Several questions re: bettas and tank?

a 20 tall isn't too deep for bettas, no tank you buy will be too deep for them.

You can include some female bettas in your tank, I would suggest 4-5 females for a tank of that size and shape.

MM

Yes you sure can place females in the tank. Female Betta's do well in groups of 3 or more. The need for this is so they can establish a pecking order. I have my male in a 36 gallon tank and he has no problem with the currents from my filter or from going to the top from the bottom. It should be much easier for the females due to not having such long fins to weigh them down.

Guppies are best kept in a group with at least three male Guppies and about twice as many female Fancy Guppies.

Good tank mates for Guppies are Cory Catfish, Small Tetras such as Neon Tetras, White Clouds, Honey Gourami, Glass Fish, Ghost Shrimp, and African Dwarf Frogs. NOT bettas! With too colorful tank-mates, such as colorful guppies and platys. Bettas, female or male, may see them as rivals, and then you have trouble in your tank.

Also, for bettas, since they beathe air, the depth doesn't matter as much as they having access to air.

Make sure there is not too much water movement as this causes stress and torn fins. A betta tank should also have live or silk plants to provide a sense of safety for a betta as well as a place to lie on, but these objects should not have anything sharp on their surface as it may tear bettas fins. Also, you have to cover the tank completely (use plastic canvas or plastic wrap to help you)--Bettas jump, and will jump out of a tank.

Good luck!

Yes you can put female bettas, but you should put at least 3 of them, that they can make their so called pekkingorder

I think you should actually be able to hold up to 5 females in there

And water can never be to deep for any fish

EB