Tried it out with one of those 1g pet carrier things. I got it to work, and have an idea for changing the air out other than just having it bubble constantly. But now I need more weight to hold it down. First I tried just putting gravel in and on it. That didnt work and resulted in it floating up when my hand slipped (there to make sure it didnt rise fast). That led to me cleaning half a gallon off of the walls, floor, and cabinet. Then I tried pop bottles full of gravel and the remaining space between the gravel full of water to make sure there wasnt air in it (not like that little amount would do much). I have family bringing down smooth rocks for use through my tank. Hopefully I can use a few to hold down the underwater land, or even cooler would be to make a structure around the underwater land to make it look better and make it stay down. We will have to see how it goes. The crabs would come later on after fish are in and happy and I can turn the water to partly (maybe fully?) brackish for the crabs.
Still considering fully aquatic ones. But I think that having underwater "dry" land would be so cool.
