I have some breeding in a 5 gal. bucket on my porch. No filter, no gravel, nothing, I actually had forgotten this bucket was there, and one day saw it and was dumping it out because the water was green, and saw tons of red ramshorns. All sizes. they must have been eating the algae, leaves that had blown in, and any bugs that drowned. Decided to leave them for the moment, and they have been in that bucket for months, the top froze over one night, but those snails are still alive and kicking.