Hi..

So I put these Clown Loaches in the tank 2 days ago, 3 of
them, to eat the snails.
I have a 20 gallon tank with 18 fish (8 of them are Neon
Tetras), Gouramis, 3 Molly's, the Clown Loaches and
others.

Sorry, personally I would _never_ recommend keeping Clown
Loaches in such a small tank. Do you possibly have any
other (bigger) aquarium?
20g tank:
Substract glas, gravel, roots, stones eventually filter
volume and look what's finally netto..

--
cu
Marco