Ph Question

do you have driftwood?

driftwood has tannic acid, which will naturally lower the pH in your water. if youre using crushed coral for substrate and driftwood as a decoration, your pH is probably swinging all over the place, and adding pH chemicals is not going to help. most fish won't notice a steady pH that is .5 off of their normal range.. what is your reasoning for needing a specific pH like that?

i don't know the definite answer to your water chemistry problem, just a likely cause.
i recommend removing the driftwood.

sorry for not elaborating. the crushed coral is a good thing, that will help your buffer and whatnot, the driftwood is what's killing you, by lowering the pH against the buffer.

messing about with your water chemistry in order to achieve a pH difference of 0.2-0.4 makes absolutely no sense. moreover, without knowing anything about your water chemistry with regard to kH, you have almost zero chance of being successful. the fact that you've not had the same "trouble" with your other rig(s) is pure luck.

it's easy to figure out why you're failing here. it's a matter of your (1) inherant kH (buffer capacity), (2) the chemical in "pH up" (3) the amount of crushed coral you're using, and (4) you don't know what you're doing.

my advice is to stop playing with your pH. whatever it is out of the tap is just fine. your doing far more damage to your fish by having an unstable pH than any good you believe you're doing by changing it.

as for driftwood, yes it has tannins (tannic acid) but it's pretty much a myth that it changes your pH. most water in the US has an inherant buffer capacity which is far stronger than the very weak acidity of the wood.