Helping A Friend with 55 gallon

My friend recently bought a 55 gal. It came cycled with 3 filters (not sure exactly what kind, but they are all HOB) and fully stocked with angels, a couple kinds of cats, and some tetras. I have not seen the tank and she's not sure exactly what she has, but it sounds (mostly based on her filter media) like the person she bought the tank from was a fairly experienced fish keeper, and from what I can tell she was not overstocked. Anyway, she said she was doing weekly water changes but after a few weeks, her fish got ich, fin rot, and started dying off.. She was only testing for ammonia, so I suggested a test kit, which she promptly bought and started using.
Ph 7.6
ammonia 0
nitrite 0
Nitrate somewhere between 80 and 160 ppm.
She immediately did water changes and bought some prime. Two days after a 50% water change and adding prime her nitrates were still in the 40 to 80 ppm range. She has done daily small water changes of about 5 gal or so but has not seen a significant change in her nitrate level. When I spoke to her, it sounded like overfeeding was a possible cause of the high nitrates, but I'm perplexed as to how her levels stayed so high after dosing with prime and doing so many water changes. She bought another tank and used one of the filters from the first tank, adding only established bio media and purigen to quicly set up a hospital tank. She began putting the sick fish in the 40 gal hospital tank... which, over the course of two days has ended up being all 10 fish she has left (3 angels, 5 tetra, and 2 catfish.) She lost 3 more fish today andmoved the last fish to the hospital tank. I sent her some ich attack, which she is now treating the hospital tank with. We are treating the fish with clean water and the ich attack first, then will address the fin rot and other issues if the need is still there.
Anywho, the question is; any other advice I should give her, or anything else we should be doing?

 

It sounds like it was overstocked. Especially based on the nitrates. If she was simply overfeeding, I would expect ammonia and possibly nitrites to also elevate. I don’t see a reason to move the disease to another tank, it’s still in the main tank, so now you have to treat two tanks. What she needs to do is as close to 100% water change as possible, and make sure she’s not forgetting to add Prime and matching the temperature (adding cold water can cause ick). Hopefully she’s thoroughly gravel vacuuming as well. It also wouldn’t hurt to check the nitrates in the tap water.

 

When I saw her nitrate level, testing her tap was the first thing I had her do. It was 0. I was also very adamant abot matching water temperatures. The hospital tank was mainly to get clean water for the sick fish because the water changes weren't working quickly enough. She was afraid to do more than a 50%, so that was the only other option I could think of. Why would the ammonia go up from over feeding and not from being over stocked?

 

When I saw her nitrate level, testing her tap was the first thing I had her do. It was 0. I was also very adamant abot matching water temperatures. The hospital tank was mainly to get clean water for the sick fish because the water changes weren't working quickly enough. She was afraid to do more than a 50%, so that was the only other option I could think of. Why would the ammonia go up from over feeding and not from being over stocked?

Because the bacteria have colonized enough, using three filters, but the problem is the end product of nitrate. If she starts over feeding then suddenly the bacteria can’t keep up with the added ammonia. Overstocked tanks can and usually will still cycle.

 

Because the bacteria have colonized enough, using three filters, but the problem is the end product of nitrate. If she starts over feeding then suddenly the bacteria can’t keep up with the added ammonia. Overstocked tanks can and usually will still cycle.

Ah, gotcha. If she was consistantly over feeding for weeks though, maybe her cycle could have caught up? She had 16 small fish in a 55 gal. The angels are the largest and they are all three 2.5 inch juveniles. I find it hard to imagine she was overstocked. I also find it hard to imagine that the water changes and prime didn't significantly reduce her nitrates, so who even knows. I'm more looking at any more advice on saving/treating the living but obviously nitrate poisoned fish. Based on her descriptions, they seem to have all manner of ailment from fin rot to some form of slime disease.