Sorry to reply back after such a long time guys. Great information and thank you for taking the time to respond. At the current, I have 2 pacus, shovelnose cat, hybrid cat, and 1 oscar. Largest fish is one Pacu at about 14 inches, the other pacu around 12in. Cats are 12-13in but skinny. Oscar around 9in. All in a standard 125gal.
I have been having no issues regarding ammonia, nitrites, and nitrates. Ammonia levels will sometimes increase at most to 0.5ppm after a large feeding, but I only do that once month or even less often, to include the catfish who do not eat many pellets. I would expect there to be a small fluctuation in pH but nothing like the continual drop that I am experiencing. Usually I feed a small amount of pellet food once a day and the bio-load does not appear to overwhelm. I have an estimated 600gph filtration, so cycling the tank about every 12 min and keeps up quite well with 6 layers of filter media and 2 bio-ball/dish scrubber compartments. Water changes are once every 2 weeks at 20-30%. I do not like to do more than that for fear of straining the biological cycle and stressing the fish.
PH meter was working, is no longer so I am using the drop test method. Less accurate but I am always in the yellow so I know pH is below 6.0. I regularly add alkaline buffer but have restrained from doing this too much since it jumps the pH very quickly and then pH drops again to under 6.0 in a couple days. Real stressful on the fish, and I have lost a few in the past through doing this. I just tested my tap water kH and it is between 4 and 5° (80ppm). When I test my tank for kH, I never see a change in color - water is resulting yellow color after the first drop. I am curious to know where the calcium goes. It is present in the tap water, but after only days does not appear in the tank.
I am planning to do a variety of tests to try to pinpoint the cause. For now, I have ordered crushed coral and am waiting for that. I realize my situation is not ideal concerning the # of large fish I have but it has worked enough to keep the fish alive and growing, even if I do have to give extra attention. I do plan to upgrade when I am able. Just to clarify however, the water quality should not be the issue because it all checks out at zero; I am instead struggling with the water parameters and continuing to wonder what happens to the calcium carbonate from my tap.
