I don't know for sure but a unfiltered tank could be Ammonia issues. Do you remove the uneaten food?
In your size tank you need to do at least two water changes a week, I would do one 50% and one 100% a week. Also the use of Seachems Prime water conditioner would help greatly for all your tanks if not filtered as that will lock up Ammonia and Nitrite to a less toxic form and you can add to the tank in drops per gallon in-between water changes.
Do you have a test kit at all to test the water for Ammonia and Nitrite? If not it might be a good idea to go buy a liquid test kit for at least the Ammonia so you can rule that out. API is a good brand for the kit, avoid buying strips because they are inaccurate. You want the ones with the test tubes and bottles.
In your size tank you need to do at least two water changes a week, I would do one 50% and one 100% a week. Also the use of Seachems Prime water conditioner would help greatly for all your tanks if not filtered as that will lock up Ammonia and Nitrite to a less toxic form and you can add to the tank in drops per gallon in-between water changes.
Do you have a test kit at all to test the water for Ammonia and Nitrite? If not it might be a good idea to go buy a liquid test kit for at least the Ammonia so you can rule that out. API is a good brand for the kit, avoid buying strips because they are inaccurate. You want the ones with the test tubes and bottles.