Have you given him somewhere to go "just above", the water line and a little way........... say 3" from the top of the tank?
They seem to like this. Going from the different area's. If they have an area below and above..JUST.....they thrive!!
I'll ask my mate 4 more info, as he love's them. Have to wait till morning though!!! I'm in Scotland, and it's 03:29!! If you tank is full to the max they may try to go further, not to escape just for air, as i said they like the option.
If you only have a BB (Beautiful betta...

, you can afford to drop a couple of inch's in depth to keep your snail happy. Just keep the waterline lower, to keep your snail happy. I'd say 4" max, to keep yout BB and your snail happy, and have them thrive, which is what we all want for out Aquababes. I'll get a hole of my mate in the morning and P.M you, if that's ok?
Hope this help's!!