couple choices... a fertilizer mix that is buffered properly... this may be enough on its own. my pH doesn't fluctuate in my 55gal rez. it's efinitely not in optimal temperature range, lol... but the microbes don't fuck with my pH. Maintain a culture of beneficial bacteria in your rez or go the sanitization route.. can't mix these, obviously but both can provide same function of keeping bad microbes in check. co2 from bubbles (it goes into solution wiht o2, too) will potentially lower pH. again, if properly buffered, this won't be enough to move the needle. microbes are another issue... if they are excreting acidic stuff, that will impact a well-buffered mix eventually. so, if you don't see a major microbial issue, change brands. if you do, implement some good beneficials or sanitizing agents occasionally... some sort of scheduled cleaning of rez etc. let the timing fit the growth.