They dont mind currents, just not strong ones. but in a 2.5gal, like mine i have heaters and no filters. if you keep on top of the water changes then you will be fine.You do need to make sure that the temperature stays constant and isnt always up and down!
id do 50% changes every 2days to be on the safe side. but getting a filter will reduce the amount of times you have to clean them.
Danny