I have the same problem in my manuelli tank.
Once I bought really small loaches that lasted a couple days and got the snail population way down before getting eaten themselves. Now the snails are out of control again.
What you can try is putting a piece of squash or zuccinni in the tank overnight and when you wake up it will be covered in the snails. Keep repeating this each night till you notice some difference.
You can also try and get some assassin snails that will eat all the other snails in the tank.
As a desperate measure you can put some copper based meds in the tank, but that will kill all inverts and yoiu will have a hard time getting the coppper completely out of the tank afterwards.