You said you don't have a filter. Is that on the 5 gallon tank? I'd recommend getting a filter, if you don't have one, and changing the water regularly (30+%) a week.
You could keep two or three snails. I wouldn't recommend getting apple snails, they can get pretty big or pond snails which are usually a pest and breed quickly. Ramshorns breed quick too, but I'd suggest them over the others.
Upgrading to a 10 gallon is always another idea
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