What is the kind/thickness of substrate you are using?
You probably have a bacterial nitrification process making your tank acidic instead of bacteria turning nitrogen into inert diatomic gas
Do you frequently siphon or poke substrate and see bubbles? (typically trapped nitrogen gas)
Ever notice a strange rotten egg smelll (that's probably the acidification from hydrogen sulfate production)?
By the time you're able to smell it is very, very toxic.
1) Keep tank heavily aerated and circulating.
2) Might want to consider a top layer of carbonate rock or shell(pure white cuddlebone in bird section of petstore).
You don't illustrate your setup, could u show us what your current tank setup is (size, substrate, plants, fish,lights)?