Mysterious Fish Deathes

Hi all!

Since about 2 weeks ago, some fish in the 60 gallon died. I tested the water and everything seems fine - 0 ammonia and less than .25 nitrite. The tank was cycled well before I got the fish. I’ve never really saw what the fish looked like before they died - until today. The picture below is a female guppy. The fish look fine, and then suddenly die in a matter of hours. I’ve lost 3 baby angels, a pearl gourami, a German blue ram, and a glass catfish, all without showing previous symptoms. They all eat happily, and swim normally. There are not many live plants in there for now, but there is driftwood, and I plan on getting plants soon. I just added a Bolivian and German blue ram, without quarantine (I know, bad choice)
I saw some spots, so I thought ich, but then the whole tank would have it? The biggest quarantine tank I have now is a 5.5 gallon.
I feed everyday with a variety of foods: Fluval Bug Bites, Repashy Superfoods, Hikari sinking pellets, frozen brine, etc. the tank is at 82 degrees, and is filtered.

Thanks in advance!

Update: the guppy just died, with I strange red patch on her tail.

Not very good with fish diseases, just don't have the experience. I will say that any nitrite is bad and suggests that the cycle may not be truly established yet. My experience with nitrite is limited as well but I had a fry tank that had elevated nitrite and I lost a number of fry before I got it stabilized at 0 ppm.

I would recommend a heavy water change like 75%, you will need to monitor your water quality until the nitrites, and ammonia are both at zero consistently.

The red patch is usually a bleed from an internal bacterial infection. Something is going through the tank as an epidemic.

Clean water may help the fish not yet infected, but it looks to me like a recent arrival was carrying something dangerous, and it has taken off in your tank.

How long has the tank been running, and how often in the past few weeks have you added fish?

How often, and much have you done water changes?

The tank has been running since the end of September (maybe the 28th or something). I added the fish when the cycle was finished. The first fish were the pair of guppies from my 20 long, the second fish were added about a week after the guppies, and they were the 3 glass cats, 2 roseline sharks, and 4 pearl gouramis added at once. Then 2 weeks later the 12 angelfish, then just last week or two, the rams. I know I should have quarantined them, and that the disease probably came from them.
I do a 40% water change every two weeks, but since the tank has a mesh lid, I have to top it off every 3 days.

Another important factor I failed to mention: the fish’s gills seem stuck open, and there bellies get sunken in. The female guppy was much bigger before.