Depending on the snails, the level they self control at is up to you, your water, your plants and some other factors. I have let
ramshorns and tadpole bladder snails run free in my oldest running nano tank. It's 5" deep, diagonal sightly more than half of a 20 long so 4 gallons plus a little. They don't get any free meals in there (no wafers or jello etc) the tadpoles pretty much self limited at about 30-40, and I've only had about 10 ramshorns survive to adult. I gladly harvest the ramshorns for my other tanks. In that tank their primary food source is melting
crypt, and the tank makes plenty of
nitrate has tons of light so the crypts do extremely well. The other stocking is 3 endler
guppy mutt females, with an occasional (like every 3 months) male added for breeding. Any fry are usually removed by 3 weeks. When there are fry I over feed and the snails do a great job of turning the cloudy food into solid snail poo and I credit my great water clarity to them.
Edit: I also have
mystery snails that get jello and wafers in some of the larger tanks, I can't comment on how the ramshorns are going to self regulate in there yet as I only added them recently.
In my big tanks (non-mystery snail) 2 ramshorns have turned into about 15. But since the tanks are very clean you can already see allot of the little guys only get so big, then starve, the shells get vacuumed out with fish waste weekly in all the tanks.
TlDr, if you don't over feed, they will trim your melting plants for you. If they starve off, make sure to vacuum the shells out. Dead snails can contribute to
ammonia spikes, but at the size of most little snails in a 20 it is negligible as long as you don't accidentally poison (certain meds, salts or copper) all of them at once. Be indiscriminate while you vacuum and your population should be ok. But up to about 10 per gallon of the tiny guys is possible so you can be the judge of whether or not you like them enough to keep them.
