mantella environment

[RobV]

I have a 20 gallon planted tank that has no one living in it right now, so my questions are:

1. it has a 2-3 gallon water area with water plants and driftwood sticking up - a total of ~5 inches deep - is this too deep of water to have in with mantellas?

2. I sometimes get mushrooms growing in the tank....bad for frogs?

Thanks,
Rob