yes its easy to keep the humidity high, but not to keep it stable, high humidity, without good ventilation will grow molds and other nasty things, you MUST have a thermostat with a heat mat, they throw out allot of heat, espcially if they are in a confined space
its not just about keeping it hot and wet, its about keeping the tank stable and healthy for the ants to live in
even European ants need good heating during the summer months, some go to extreme lengths to do this (Formica mounds are a good example)
Highly recomend you spend some time looking at ant nest construction in the wild, and the difrent requirments they need, will help allot in the choices you make for your formicariums
