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pH, GH and KH

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Warning!  This is a very long post. You may wish to skip the whole thing if you don't want to read about pH, KH and GH!

Appeal! If anyone reading this has suggestions for inclusions, deletions or other edits, please PM them to me so I may incorporate them. With co-operative effort, I'm hopeful that  Clothahump may consider the result worthy of adding to the Articles section.  All who contribute will, of course, be acknowledged.

I have completed the first draft of the section on pH and invite comments (preferably via PM). I will add the other two sections, as they are drafted, in separate posts

I'm no chemist, so what I say is based on my understanding through reading and (somewhat limited) experience. Also, for the sake of clarity (I hope!) I've made some simplifications to the following...


This is the standard way of measuring how acidic or how alkaline a liquid is. The 'neutral' point is 7.  It's a logarithmic scale of measurement (like the Richter Scale for earthquakes) ; so, for example, the difference between pH7 and pH6 is not a factor of one unit of acidity, but of 10!

Why Is Knowing About pH Important To Us?

Nitrogen Cycle

It so happens that the bacteria we need in our filters are sensitive to pH. They become inefficient at pH6.5 ; and if the acidity drops to pH6 they usually shut down altogether.  When they shut down, the cycle "stalls".

Similarly, if a tank's pH is allowed to increase to a very high value, the cycling bacteria will shut down.  The optimum pH range for a cycle is roughly between 7 and 8.

Once a filter is cycled, a PH 'crash' (where the pH falls to a low value in a short space of time) can knock out the biological filtration. However, it does seem possible to lessen the inhibitory effect of low pH on bacteria simply by adding clay to the system!


Ammonia in an aquarium exists in two states : ammonia itself ; and ammonium.  The former is extremely toxic to fish in very small concentrations ; the latter not so.  The ratio between the two that is present is dependent on the pH of the water.  At low pH, a greater proportion of ammonium will be present ; and, conversely, at higher pH a greater proportion of ammonia.  Ammonia test kits usually report the concentrations of both, combined.

It follows that we have to be more vigilant about ammonia in our tanks if the pH is above 7.

Selecting Fish For, and Introducing Them To, an Aquarium

Natural fish species have each evolved in specific aquatic environments and their metabolisms have developed to operate in those environments. They may be intolerant to different water conditions, even given time to acclimatise ; they will certainly be intolerant to water conditions that change too quickly.

pH is one of the factors (water parameters) that influence fishes' physiologies because it has a direct effect on the way their internal chemistry works, particularly so far as blood chemistry is concerned. Fry can be particualrly sensitive to adverse pH conditions. It follows that we should choose fish that will be healthy in water of a pH that we can provide. For most of us this will be tapwater, so it is important to know the natural pH of our tapwater before selecting fish.

The difference in pH between an LFS's water and our tankwater is also significant. If the difference is severe, then without a period of acclimatisation to our tanks' conditions, fish may die of 'pH shock'.  The greater the difference, the longer the period of acclimatisation needs to be. 

Many sources hold that the pH of a fish's environment should not alter more than 0.2 of a pH unit within 24 hours. If a fish is introduced into a quarantine tank at home, then the pH gradient can be managed over a long period. But most hobbyists do not have quarantine arrangements and acclimatise their new arrivals in the transit bags. In these circumstances, we should take great care with the acclimatisation process ; where there is a large pH difference it can take many hours and, even then, the steepness of the gradient is likely to be less than ideal for the comfort and health of the fish.

Effect of pH on the Tank Ecology

Similar relationships exist between pH and the health of other larger organisms in our tanks - crustacea, snails and plants, for example.  Tankwater of low pH may chemically erode the shells of crustacea and snails, causing their early deaths. Some plants have a definite preference for either high or low pH and, as with fish, choosing the right plants for the conditions can be a significant factor in whether they are successful or not.

But as well as larger animals, our tanks are hosts to a huge range of micro-organisms which all play their part in managing a successful biological system. A tank is said to be 'mature' only when a healthy population of micro-organisms has been developed and has coated all the tank's internal surfaces. As well as having a biochemical role, micro-organisms (sometimes referred to as 'infusoria') act as a food source for fry and may be an important part of the diets of some 'grazing' fish.  Since it is reasonable to assume that some, or all, infusoria may be sensitive to pH, [editing note : anyone have information that confirms this?] this parameter may influence a tank's ecology in general and certain fishes' nutrition in particular.

Relationship with KH

There is a direct relationship between pH and KH which will be discussed (to some extent)  below.

[Anything else I need to add?]

Very good mate, I've stickied it for you :)

Did this ever get finished up? Husband's degree is in mathematics with ocean sciences and he has a bit of knowledge on water chemistry. (I know, freshwater aquaria are very different, but he does know about this stuff!)

Oooh!  Would he be prepared to collaborate, do you think?

How did I miss this, looks good and well worth putting in the articles section.


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