If you have South American fish species, they usually require low pH, which is ideal for cacti.
Most of the fish for sale today have been bred and acclimated to more neutral or high pH water and do fine. I've been keeping South Americans and other low pH species in my hard alkaline (dechlorinated) tap water since the early 80s, and I've never once even attempted to adjust the pH. They all do fine. Of course I don't keep neons, cardinals, or discus... Mostly the big bruiser cichlids and cat fish. Right now I have Salvinis, Jack Dempsy's, Convicts, Leporinus, pink tailed chalceus, some corey's, and a big pleco.
Okay enough highjacking. Lets go to the "other pet" section if you want to talk more. Sorry Rodney.