Help with High GH, Low pH, Low KH

More info on the "alkaline buffer"?

What is your tap water like in terms of hardness/TDS? I would use something like crushed shell, coral, or aragonite substrate to try prevent the pH dropping. My tank is run on rainwater (no mineral hardness) and has a large quantity of driftwood, and a small amount of crushed shell [beach sand] in the substrate is enough to keep the pH stable around 7.

Due to your low kh (carbonate hardness), your ph isn't stable and keeps dropping. Increase your kh. Here is a quick read: http://www.theaquariumwiki.com/KH

duanes said:

I just googled the Moline water quality report, and tap water has a pH 9.4, but hardness and alkalinity are on the low side.
So because buffering capacity is low, you will need to do very large, frequent water changes to keep fish urine levels low, maintain stability, along with the suggestions by above posters. I'd be doing 50% water changes every other day with that stock, and that size tank, and would (along with a crushed coral substrate) add a large fluidized bed reactor, using aragonite sand as bio/buffering media.

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That sounds excessive to me, although you're right he does have a heap of large (and probably getting larger!) fish in there. Aside from the issue of keeping the nitrates in check, I would have thought large water changes are the last thing he would want to do given his tap water has very low hardness and [in theory] with the reactor/substrate/crushed shell etc his tank would have higher hardness. Every 50% water change would be dropping the mineral content of the water by nearly half, until the buffers bring it back up. IMO smaller more frequent water changes are the key to stability over large (50%+) changes. My current tank isn't really a fair comparison as it isn't anywhere near as heavily stocked as this, but while I was building it and had all the fish in a 400L I didn't have any problems with water parameters (nitrates or hardness/pH) with a weekly 30-40% change and a bit of shell in the substrate, and as I said before my tap water is about as soft as it gets.

Perhaps it would be worth while investing in a 55g drum that the tap water could be conditioned in with a bag full of crushed shell before being added to the tank? Although with that stock I dare say the best investment would be a larger tank...

Very good point.

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I would like to hear what the water change schedule is like? If the fish mentioned are good sized that's a fairly high bio-load. IMO the best way to combat water issues is more frequent and or larger water changes. Most people don't change nearly enough water. Not a big fan of adding chemicals as a way to "fix" water even if it can be done outside of the tank, its just to hard to get a stable and consistent condition. Most fish will adapt to your water as long as its clean and the parameters are consistent. I have very high bio-loads in most of my tanks and I try to do 40-50% at least twice per week, every other day if I have time.

Low PH seems to be a pretty common issue for people that have huge bio-loads in relatively small tanks.

The solution to this problem is stocking the tank more appropriately. If the tank has been running for 1 year and assuming the fish are 1 year old, the bio-load is way too high for a 125G tank. Maintaining healthy water parameters with that stock is going to be a losing battle.