Hmmm, I don't have an answer for it either 
Well at least it's a good sign that the pH is under control and the fish are healthier.

Well at least it's a good sign that the pH is under control and the fish are healthier.
Hi Nick,Nick660;1429078;1429078 said:Ahh...you managed to get hold of a KH test kit.
Looks like your tap water KH is relatively low and certainly not enough to maintain a stable pH in a tank with a heavy bioload! What little KH is going into your tank during water changes is being quickly consumed allowing the water to acidify.
If your tap water pH is stable at 7.5 then the easiest route IMO is to maintain your tank pH at the same level by adding bicarbonate of soda.
Use the calculator I mentioned previously to work out how much to dose http://www.dataguru.org/misc/aquarium/calKH.asp
You'll need to go carefully to begin with as pH will go up in big jumps when the initial KH is very low. Put some figures into the calculator and you'll see what I mean!
You may have to accept a big jump to begin with if your KH is truely at zero. The problem will be that before you have a chance to dose with bicarb on the next day, the KH may be back to where you started. If you stop feeding and use strong aeration it will help reduce the amount of KH being used up.
You'll certainly have to dose with bicarb daily until you can get KH above 4.5 and continue to keep it there.
I thought you added crushed coral to the tank? This should have helped provide some KH to a certain extent? Crushed oyster shells in the filter are another option. Anything to stop the KH slipping back down after you raise it a bit each day.
Large daily water changes might help but from looking at your test results this would still cause big swings in pH and alone would still not provide enough KH in my opinion
If the fish are suffering then you may have to bring the pH up quicker than what would be considered safe but that's your call.
Good luck.