Sometimes tanks do weird things...
I had a tank, and it was bordered by an exhibit with a fence. When I later tried to expand my tank, the tank suddenly lost all its water and the fish were flopping about on the land. @___@ So I turned it into a hybrid tank... okay, since I had turtles in there too. But then I added a keeper gate to the side which had been touching the fenced exhibit, and BAM, it turned back into a tank again. Except the water level wasn't up to the top of the acrylic walls, it stayed down at the level of the land. So I'm like, "Whatever! I give up." and went to work on something else.
Strange.
I DID, however, finally figure out how to make a hybrid exhibit that connects to a show tank. You have to use the "create ditch" tool in the landscape panel and sort of shave off the bits of land that stick up beside the tank wall. Then you can add a tank portal.