Slow death, few symptoms

Thank you in advance.

I have 2 tanks, parameters as follows- same problem in each tank: 75 gallon: temp 75 F, pH 7, Ammonia 0, Nitrite 0, Nitrate 0. Loaches(zebra, rainbow, skunk, red tail), angelfish, acaras, synodontis, planted. Set up for 7 years, moved several states successfully.

65 gallon: temp 75 F, pH 7, Ammonia 0, Nitrite 0, Nitrate 0. Endlers, rasboras, stiphodon gobies, gudgeon, otocinclus, Cory’s, khuli loaches, horse face loaches, hatchet fish, hill stream loaches, mystery snails, merits snails, planted. Set up for 2 years, was an upgrade from a smaller 20 gallon tank that had been running for 3 years previously.

Each tank recieves a monthly 60% water change, I’ve been doing water changes this way for years, long before the problems began. I treat tap water (Albuquerque NM) with Seachem prime and let it sit in a thoroughly cleaned bathtub before each water change. I make sure the tub is rinsed thoroughly with fresh water beforehand and no chemical cleaners have been recently used.

Between the two tanks over the last year and a half I have lost 2 zebra loaches, 2 skunk loaches, 1 African butterfly, 1 plecostomas, 1 acara, 5-6 Cory’s, 4 khuli loaches, 1 gudgeon, 3 horse face loaches, 3 yoyo loaches, 5 hatchet fish, 4-5 Otto’s, 1 hill stream loach, and several golden barbs(probably old age).

Symptoms: Prevailing symptoms are fish, especially loaches, become skinny then die, usually over several months. The cory’s fins deteriorate before they die. The synodontis is still alive but is often breathing heavily. All fish continue to eat voraciously up until their death, even though they become thin.

I recently added 3 more hatchet fish to the 65 gallon and they all died within 48 hours, followed by a gudegon, 2 khuli loaches, and 2 cories, and a hillstream in the same tank. No apparent symptoms, just dead, not even wasted away this time.

I have suspected parasites and dosed each tank with Prazipro followed by Paragaurd on several occasions, but am unsure if that helps. Currently the remaining zebra(6+ years old) in the 75 gallon is wasting away. The remaining Cory in the 65 gallon (sadly alone now) has deteriorating fins.

I feed a mix of dry foods (flake, sinking, floating) several times a week, depending on the eating habits of respective residents.

Are there any parameters that could cause this I am not controlling for? Should I worry about gH or kH. Would metals.. copper, iron, lead, etc be an issue? Albuquerque water has. Trace amounts of uranium, chromium, and arsenic.

Each tank has a Fluval FX canister with plastic mesh in it for a robust biological cycle. No carbon or chemical resins. I dial back the flow in the tanks a tad to keep strong currents from being an issue, still plenty of flow though. Each tank has an air stone as well.

I also have a 2 gallon planted tank with cherry shrimp, 2 scarlet badis, and 1 stiphodon. No issues in that tank yet. 2 small sponge filter in that tank, same water change regime.

Any advice, or suggestions for new areas of inquiry appreciated.