I have a very healthy bladder snail population in a 20L, and, as much as I hate to do it, I have to cull. The best two things that attract them in a group so far is blanched zucchini and dried mulberry leaves. This aquarium was established in March with live plants (thus snails) and with fish in late April.
I bought the mulberry leaves in a collection of dried botanicals. Just added one yesterday. The snails have annihilated it already. I know it was them because I saw them. I could have taken it out this morning, but I didn't. I'll do it sometime, probably tomorrow after I add another one. The first is totally gone already.
I add a blanched slice of fresh zucchini on a fork for my otocinclus (obtained in mid May) every other day and leave it overnight (who fortunately have finally found it) and in the morning it is rife with little snails. I've culled via zucchini once.
I honestly hate doing it, but my aquarium is so nutritious that my initial population has exceeded my wants. I definitely don't want *no* snails! But I provide such good food for the shrimp, otos, and fish that I have to cull now. I feel so bad about it. I know many of you know what I mean.
Someone will say "give the to a store or other hobbyist" but I am in a small town in the middle of an aquarium store desert with little interest as far as I know. I'd consider getting an assassin snail, but then what? I can't have botias because it's just a 20 gallon. Such is the life and angst of an aquarium keeper :/
On the plus side - I have a thriving otocinclus group of 6 - and I want more now - when I was SO worried about having them since my past experience with them made me wary, thinking they were delicate. They may be, but mine love all the stuff on the glass, and the wood, and the plants (all live) and again, the zucchini!! Their bellies are so round lol
My best successes so far are the otos and the plants. I can't take credit for the snails - they are invincible.