A mystery snail can't live with a betta in a 2.5 gallon tank. It's too small, they get big and poo tons. Lol
1 mystery snail does best with 2.5 gallons to him/herself.
In a 5 gallon with a betta would be good.
I have my mystery snail in a divided home with one of my bettas, no filter, but I do daily water changes plus I try to take out any poo I see when I go visit them. They both have live plants.
Fast growing plants are best for aiding the water quality.
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