You need to provide them with land as well as water. Mine have half a sandy area where they can hang out, and it also has a water part, only about 5-inches deep, brackish water. They spend most of their time out of the water, but like to hang their tails in the water.
It's also very important to keep up the humidity. If the air temperature gets too cold, they will go dormant. They need very warm water, and very warm air. I have an alarm installed on my tank that goes off if the tank gets too cold, or too hot.
Mine sit on the filter, and the heater (make sure your heater has a plastic cover so they don't get burned). And make sure they can get out of the water easily. Strangely enough, Mudskippers are the only fish that can actually drown!
They are amazing creatures, that's for certain. I have fallen in love with mine though and would not go without them.
