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How to maintain water hardness level?

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jacksdad:
The water where I live is very soft, Gdh is 1.91 (according to the water supplier).
I learned on here that the gravel in the tank can alter this, so I got a hardness tester, tap water is CacO3 60 ppm, my tank water is 80ppm.
I've put a bit of gravel in a food tub, with tank water in, today I tested it, after soaking for 5 days, its 140ppm!
Now, if I do a 50% water change every week, the water hardness will drop, then climb again over the next few days: isn't this bad for the fish?
Its not a huge change, but I believe fish don't like water hardness changes, which has me wondering whats best: clean water or leaving things be, and testing for nitrates?
As I've said before, last time I had a community tank was about 12 years ago, from memory I did 25% water changes about once a month, and the fish were doing well, with swordtails breeding like bunnies!
Could the reason be that the tank water was harder than tap water, due to being left alone?

Stephen:
Mike
I had the same issue a few years ago.
I go rid of the gravel and replaced it with sand my water hardness is pretty much constant now.

60ppm is approx. 3.3dGH whilst 80ppm is approx. 4.4dGH, 140ppm is approx. 7.8dGH (dGH = degrees German hardness)
It is quite likely that the previous swordtails & guppy survived due to the hardness in the tank.

jacksdad:
I think I'll have to remove the gravel: I also tested a tub of the unused Limpopo black sand, after 8 days the tank water is still 80ppm. As you can see, the results are probably higher than the tank itself as the water/substrate ratio is about 50:50, so hopefully removing a lot of the gravel and adding the Limpopo will sort things out

Stephen:
The Limpopo sand will be fine.

jacksdad:
Yes, the Limpopo sand is 100% inert, I tested it!
The gravel is definitely making the water harder, so today I have done another 25% water change, before adding the fresh water I removed just under 3 Kg of gravel. Wasn't easy as I have added the Limpopo sand on top of it.
I just scooped up as much gravel as possible without uprooting the plants, then straight away dropped in the sand, managed to keep all the plants in place.
As for the gravel/sand mix I removed, I simply passed it through my kitchen sieve. I now have the 3Kg ish tub of gravel, and about the same amount of Limpopo that fell through the sieve, in 2 tubs.
I'm going to leave them both for a few days then do a hardness test, hopefully the filtered Limpopo will be inert, but its obviously got the fine gravel mixed in.
I've still got 5Kg of clean Limpopo sand as I bought a 25 Kg sack, I may well end up removing the gravel/sand mix, then adding clean sand, until the hardness returns to tap water level...keep watching!

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