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Oak Leaves

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                                                                                                                                                                                                             If you go down to the woods today ...
I've been asked, off-stage, if we could have a 'sticky' regarding the use of oak leaves in our tropical freshwater tanks.  As it's that time of the year it seemed appropriate - indeed the Member who made the request had just returned from an expedition with a bin-bag bulging with them. And given that the request was pointed at me, it seems appropriate that I kick things off! There are numerous mentions of oak leaves buried in the forum (where you may find bits that I've missed!) but I agree that it would be good to be able to reference a single thread.
 So what follows is my draft offering and is a mix of fact, opinion and pointers for discussion and further research (if interested!).  Because it's based on my own experience, undoubtedly there'll be info that I'll have missed ; so if you have things to add, or if you'd like to point out inaccuracies, please do so. 
 Other leaves are available of course - such as beech and, more exotically, Indian almond and teak ; but oaks are relatively plentiful and their leaves appear to disintegrate less quickly in a warm aquarium than those of other potentially useful native (to the UK) trees.  So unless I make specific mention of other leaves, all of what follows concerns oak. And cold, wet knees.
 My standard way of evaluating what oak leaves etc. do to water is to immerse 100 grams of them in 10 litres of water. For practical purposes, in (say) a 100-litre tank, a handful might be used ordinarily. A handful that has been stored frozen might weigh around twenty grams.

 Oaks shed their leaves rather later than other trees and their eagerness to do so depends on latitude. In Scotland, for example, the end of October may be a good time to go 'leafing' (as Mrs. V! puts it) ; in Southern England it may be mid-December. In Milton Keynes I usually wait until the beginning of December but, energised by reports of bulging bin-bags brought back from about twenty miles North of here, I went out a few days ago to inspect 'my' tree.  This is it ...
 ... and it was still more or less fully dressed.  The ground under it was covered with (mainly) beech leaves from neighbouring trees ...
 ... so the start of December still looks like a good bet for me!
 The leaves we use must have already fallen deciduously from their trees and be brown (green ones brought down by hurricanes don't count!).
 Even though we're likely at least to wash the leaves we collect, it seems sensible to select trees that are distant from exhaust fumes and/or other potential nasties.  There was a study that suggested deciduous trees use leaf-shedding as an opportunity to rid themselves of toxins that the tree may have accumulated during their growth periods : so it may be wise not to choose a tree growing near a copper mine, or some such location!
 And if the chosen collection point isn't a 'public' area, it may be wise to get the landowner's permission to fossick around on his or her property.  'My' tree is on land owned by the local Parks Trust, which readily agreed to my stealing leaves and small branches that had fallen.


Whether the collection method is by picking one leaf at a time or by doing an impression of a JCB, there may be things on, in and around the leaves that we may prefer to exclude from our tanks. In addition to the possible surface pollution already mentioned, oak trees are a haven for all sorts of bugs and pests that have evolved to cope with the leaves' tannin content : in fact, so riddled with them do they become that in Summer oaks produce a second flush of new leaves.  There may be fungi, creepy-crawlies, grass, other leaves/plants and soil mixed up with what we gather off the ground which may or may not pose some kind of threat to our tanks' residents - but I prefer to err on the side of caution and so I clean and semi-sterilise the leaves before storing for future use.
 I dump batches of leaves in a basin of tapwater, give them a good swoosh and then pick out the leaves (or pick out the detritus that I don't want, depending on relative amounts!).
 From there, the leaves are transferred to a large pan - actually my marmalade-making preserving pan - of simmering RO (though tapwater would be fine), brought back to simmer and stewed for about a minute. They're lifted out and drained and are then ready for whichever storage method is to be employed.
 They can be very gently oven-dried and then packed into sealed bags to be stored in cupboards or sheds ; but I allow my leaves to cool before packing in bags and freezing. I get 200 grams of leaves in a large Tesco zip-close freezer bag.  Although it (obviously) entails extra expense, I prefer the convenience of having the leaves sink quicker when I put them in an aquarium.  I get fewer broken leaves, too - though aesthetics isn't really a prime concern of mine!
 The liquor that is left in the pan is essentially blackwater extract - stuff that costs a bomb from an LFS.  I cool it then pour it into plastic milk cartons and store in a dark place ; if stored in sunlight, algae live and die in the cartons and can produce ammonia. I then use it through the year for conditioning water-change water. If it runs out before the end of the year, I either sacrifice some frozen leaves to make some more or I source a batch of Indian Almond leaf 'chippings' relatively cheaply from Thailand and use those.

The leaves, whether dried or frozen, don't sink straightaway, so it may be preferable to waterlog them separately before putting them into a tank.
 Frozen leaves take a couple of days or so to sink at room temperature, and a day or so at tank temperature ; I can't now remember for certain the equivalent time for dried leaves, but I think it was about a week.
 The leaves' acidifying property will be exhausted after about a month in-tank, by which time they will have begun to darken in colour ; eventually they'll turn black but will typically take much longer to disintegrate than (say) Indian Almond leaves - which degenerate to skeletons before they have a chance to go black!
 In common with oak bark and some bogwood, oak leaves contain a deal of colloidal material – this is the stuff that generates that slimy/furry substance that sometimes exudes from bogwood and gets populated with fungi and bacteria (until eaten by fish etc.).  However, unless a small tree’s-worth of leaves are being used, it shouldn’t be noticeable.
 Whether it's better to remove and replace 'exhausted' leaves, or whether to add more and let the old ones disintegrate, depends largely on circumstance and/or keeper preference.
 For example : if you have a spick-and-span 'lounge' tank with sparkling white sand, having it constantly messed up by the black grot of decomposed leaves may not be particularly welcome!  In that case, all other things being equal, you may choose to replace all leaves before they got to that stage.  Conversely, if aesthetics aren't a concern and the creatures supported by decaying leaves are useful, you may wish to maintain a certain proportion of grot for their benefit.
 Hopefully some of what follows may help further to inform keepers' individual decisions.

The acidifying effect is generated by tannic and humic acids, and it starts immediately. Whether or not we notice it depends on the quantity of leaves relative to water volume, and on the buffering capacity of the water. In a tank’s volume of hard water, with a high concentration of bicarbonate, a handful of leaves is unlikely to have effect that’s measurable by a liquid pH kit ; in soft water, it almost certainly will.
 In one my ‘standard’ trials (100g leaves in 10L water) with water that started out at KH 11 and pH 7.4, the pH had fallen to 6.7 after 24 hours ; three days later it was 6.5 . It then fell gently over the following (nearly) four weeks to 6.2 before showing signs of rising.
 In a similar test, this time using water at KH 0 and pH 5.8, pH had fallen to 5.3 after ten minutes ; three days later it was 4.9 . {NB: this test is still running as I type this, so I will come back and update as and when}.
 The reversal in the downward trend of pH (in the case of oak leaves in ‘normal’ water, after about four weeks) was echoed by other ‘acidifying’ natural materials that I tested, sometimes much more noticeably. I was intrigued to find a supporting reference reporting the potential of such materials to counteract the environmental effect of acid rain whilst they decompose! Perhaps (I thought) there are then two stages in decomposition: an initial one in which the contents of, say, leaf veins are discharged into the water – resulting in acidification ; and a second one in which cellulose breaks down - releasing more alkaline cell contents?
 I’m trying to test in a second trial running alongside the one I just mentioned. Not having (I don’t think) the chemicals necessary to rupture cell walls without affecting tests, I took the rather brutal approach of macerating 100g of oak leaves in a kitchen blender, hoping that at least some damage might be done on the cellular level. So far, (day 4) both sets of results are identical ; but maybe I’ll have hurried the process along a bit, so if a differential does appear over time I’ll report it here.
 Another explanation for the pH reversal could be the release of minerals (see below) ; in particular – phosphate.  Those pH-adjusting preparations, with names like ‘Proper pH 6.5’, are based I think on equilibria between different forms of phosphate. It seems possible, but I don’t have enough knowledge or understanding to comment further!

Other (Inorganic) Influences on Water Chemistry
The release of minerals from a handful of oak leaves immersed in a tank’s volume of hard water is unlikely to be directly or indirectly significant to fish – especially where plants fertilisers are being introduced. But, again, the more soft a tank’s water is, the more valuable they may become.  In extreme cases where fish are kept in otherwise pure reverse osmosis water, they’re the only minerals around (bar those contributed via the fishes’ food, of course).
 I thought I’d test, so far as I am able, to see what contribution oak leaves could be making to the mineral content of one such tank (holding Licorice Gourami) based on my 100g-in-10L ‘standard’ after the leaves had been immersed for two days. The generally-presumed normal imprecision of hobby kits was compounded somewhat by the presence of tannins, colloids and colour, so perhaps the following results should be viewed as more indicative than might be the case in more usual conditions - apart from TDS, conductivity and redox, for which I was able to use electronic meters.
* TDS (Total Dissolved Solids) had risen by 47mg/l ; and conductivity by 66 µS/cm.
* Redox (Reduction/Oxidation Potential) had shifted from the +113mV of the ‘raw’,aged RO to +101mV.
* A GH test seemed to indicate that calcium/magnesium was far below 1mg/l.
* A Salifert carbonate test, rather disconcertingly given the low pH, showed non-zero (but not more than 0.3mg/l).
* Ammoniacs were negligible. Total Ammonia : zero ; Nitrite : <0.05 ; and Nitrate : c0.03 . I expect much nitrate is generated in-tank from digested fish-food (they will eat only live foods).
* Phosphate was off the scale. A 10-x dilution gave a result that made some sense (though not perfect due to the ‘muddiness’ caused by the other content of the sample) and after multiplying-up was an estimated 8mg/l.
I have a potassium test which is difficult enough to use in ‘perfect’ conditions, is past its ‘use by’ date and has been stored incorrectly for the past year. Nevertheless I gave it a go and, unsurprisingly, it returned an unfeasibly high result! It seems likely, though, that the leaves would be leaching an amount of potassium along with the phosphate.
 Thus it appears that decaying leaves are supplying significant nutrients to the rather desperate plants in my own Licorice Gourami tank – something we might all have guessed, of course. The plants, in turn, are contributing to the system in their own ways.
 Used in usual quantities, then, I don’t expect that oak leaves have measurable effects on water chemistry in most aquaria, apart from phosphate, maybe.

In the commercial world, tannic acid seems to be a specifically-formulated substance with the formula: C34H28O21 and a molecular weight of 772.57. In nature though (so far as I can determine) it exists as a number of structural variations on the theme. It’s an organic acid as opposed to an inorganic acid and I've seen it described as a mixture of large- and medium-sized molecules. Some of the notes in this sub-section also apply to the humic acid that leaches from oak leaves.
 The most obvious effect of tannins is on the colour of the water. In 'blackwater' aquaria this is perhaps no bad thing because it reduces slightly the intensity of light through the water column. In doing so, and perhaps in combination with floating plants, rooted plants and decor, they make a contribution to fishes’ feelings of security. Many keepers object to their tankwater being stained though and some may think to remove it with activated carbon ; however carbon will remove not only the staining but its acidifying property along with it (I’m fairly sure). But it should be possible to control the staining (if it's building up too much) by running activated carbon every so often, effectively ‘resetting’ the tank in that regard.
These first two photos below show the colour obtained from 100g of whole leaves in 10L of water : on the left after ten minutes ; in the middle after 24 hours.  The photo on the right shows a comparison between pure water and a dilution equivalent to 20g of leaves in 100L of water ...
And as your bonus for struggling through thus far, the next photo is of the 'blitzed' leaves, with the liquor having been passed through a sieve (53-micron).  Notice the murkiness cause by the colloids released from the leaves ....
Tannins are able to bind with, and precipitate proteins in the water column : a kind of freshwater skimmer, I suppose. However unless they’re removed somehow once precipitated, they must be disassembled by bacteria and will therefore still contribute to a system’s nitrogenous load.
 Tannins make their greatest contribution to tank health via their anti-bacterial, anti-fungal and anti-viral properties which are active towards a range of in-tank pathogens. In a ‘biotope’ tank, where decaying leaf-litter may be plentiful, the native pH is usually hostile enough. In a typical community tank, though, these properties have the opportunity to act alone : the greater the concentration of tannins, the more effective should be these properties irrespective of the prevailing pH (that is an assumption).
 If tannins are anti-bacterial, though, do they therefore negatively affect our nitrifying micro-blighters? Thankfully, according to one study (included in the references below) probably not : neither ammonia-oxidising bacteria nor, more relevantly to most of us, ammonia-oxidising archaea in the soil samples used were found to be troubled.
 Tannins are degraded by bacterial action over time – bacteria, presumably, which are resistant to the antibiotic property – and are adsorbed by products such activated carbon and Purigen. 

Microbial Support
As leaf-litter degenerates it becomes host to a food chain involving creatures ranging in size from bacteria, through infusoria to invertebrates such as shrimp and on to fish.
 Many post-mortem reports that discuss the contents of fishes’ stomachs cite the amount of vegetable detritus that they contain. In foraging for invertebrates it’s to be expected that a certain amount of rubbish will be ingested, but decomposing plant-matter may have intrinsic benefits. The roughage contribution to diet seems obvious (c.f. pea-feeding!) but it seems that bacterial action may well change the nature of the fatty acids contained by leaf-litter into forms that are more nutritionally beneficial. Some creatures are known to detect this change ; it’s not beyond the reach of (my) imagination to suppose that some fish may, too.

Residential Leaf-Litter
Many fish live among leaf-litter in the wild : it affords protection and offers a ready supply of food, particularly for fry. All an Apistogramma needs is a leaf to defend and it’s got a home, seemingly. Indeed there are field reports from Amazonia of random handfuls of leaf-litter containing writhing masses of Apistos! Most of my Licorice Gourami live under leaf-litter, each with its own territory and tunnels to the ‘outside’ for use when food appears!
 It’s somewhat difficult for most of us to emulate such an environment in our tanks - especially perhaps if they’re ‘lounge’ tanks. But I wonder if a set-up that included (say) a discretely-buried Tupperware container full of leaf-litter may invite some fish to exhibit such natural behaviours.

The contributions that leaf-litter can make to a freshwater tropical aquarium appear to be overwhelmingly positive, but are there potential difficulties, too? This sub-section flags up some that have occurred to me ; there may well be others. So if you’ve had the stamina to read this far and can think of additional ones, I’d be very grateful to hear of them.
 Some of these potential concerns are a function of maintaining ecosystems in our (relatively) small closed, recirculating glass boxes, where continuous natural refreshment of the environment is absent. As with other aspects of tank design, it’s a question of balancing leaf-mass with an aquarium’s size and bioload.

* The myriad of creatures that are supported by decaying vegetable matter use oxygen. If ‘old’ leaf-litter is allowed to accumulate, the negative effect on a tank’s dissolved oxygen concentration is potentially significant, particularly where a tank is well-stocked with fish. So in such cases, we should ensure that the water is well-oxygenated and, if appropriate, regularly remove exhausted leaves. With a low stocking density and a ‘normal’ amount of leaves in a tank, though, impact on oxygen demand is less of a worry.
* The material that leaches from leaves also adds to the amount of dissolved organic compounds (DOC) in the water column. These feed bacteria which, again, impact on dissolved oxygen. However, the action of these bacteria break down DOC into inorganic compounds that can then be absorbed by plants. In turn, plants are nett oxygen contributors during times when most fish are most active. Under normal circumstances, though, regular water-changes of sufficient volume is the way to manage DOC.
* I mentioned above that by shedding leaves, trees may rid themselves of toxins that they might have accumulated. In fact the source for this, Brian Ford, states that the concentrations of heavy metals increase ‘a thousand-fold’ immediately before leaf-fall. This seems of little consequence to those of us who use good dechlorinators in-tank, but it’s potentially an issue for others who use reverse osmosis water or, maybe, who dechlorinate with prescribed dosages out of tank : successive batches of leaves each could deposit heavy metals that may become problematic over a long time. I wonder, therefore, whether it may be advisable for the latter two groups of keepers to include periodic in-tank dosing of dechlorinator in their management of leaf-litter if they’re allowing it to accumulate and decay. I may well be being paranoid here - I will try to find out more about this.   
* In addition to harbouring bacteria that are largely beneficial, accumulated leaf-litter may also provide a haven for less desirable organisms that may not be affected by tannins’ medicinal properties. So if a tank is known to be affected by a parasite, it may be advisable to withdraw the leaf-litter until the all-clear is given and then replace with fresh material. A typical case in point would be whitespot disease.
* It’s reported that in sufficient concentration tannins may have unwelcome effects on aquatic life, compromising respiration and metabolism – I’m unable to get to the original paper in order to check whether that means or includes fish.  However, it’s unlikely that we’d be contemplating a deep bed of leaf-litter in a tank of fish that don’t prefer soft/acidic/blackwater conditions, but it’s as well to be aware that (for example) guppies may find water that is highly loaded with tannins somewhat stressful. Of course, a few leaves in a decent-sized tank shouldn’t be a concern.   
These are some of the sources I looked at while writing. Mostly it was checking that my experiences with and (very flaky!) knowledge about oak leaves had some external support but, as always, I came across other interesting bits and pieces not necessarily used in my text. (attachment)

Terrific article! Thanks

Great Andy. Put this article as a sticky at my Iphone at least !!!!

Great article, thank you for taking the time to write it

Very good read. Thank you for taking the time and effort to write.



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