Sick aquarium? Fish dying off over time

[Feawen]

Hello

We have a 55 gallon community tank which has been set up since July. Over the last couple of months our fish have been slowly dying off and I don't understand why. I do a partial water change once a week and vacuum the gravel. I'm new to keeping fish so I don't know what signs I should be looking for. We are down to 2 platys, one danio, 3 upside down catfish, and a ghost shrimp. We also house an undetermined number of red ramshorn snails (whoops) and 7-8 assassin snails who are very well fed.

Levels as of this morning:
Ammonia 0
Nitrite 0
Nitrate 5-10
Ph 7.8
Temp 78

Did a pwc today after I took the levels. We have a marine land 75 gallon biowheel filter, and I've experienced ich and know how to deal with it. They don't show signs of it. Please help! Oh we don't have any live plants either. I don't know what other information I can give...

We appreciate any help you can give us!!

 

[Mumma.of.two]

What do the sick fish do before they die? Any signs like fuzzy patches, listlessness, sunken bellies, etc?

 

[Feawen]

I haven't noticed any sunken bellies but they are definitely listless. They basically stay hidden during the day and only come out when I feed them. No fuzziness. They do eat though.

 

[GNarnian]

Do they look super thin? Thin for fish I mean. I know you said they are eating, but do they look skinny?

 

[Feawen]

No, as far as my husband and I can tell they look normal. You just don't see them swimming around unless food is out.