If you want the tanks connected to where the fish can travel back and forth you will have to make sure both ends of the connecting pipe are below water level and full of water, if you get air in the tube they cant go back and forth. Might be kind of hard to get the tube filled with water and no air trap in it. If you run co2
or air bubblers in there the air or co2
will eventually get into the tube and displace the water. i cant honestly think of a good way to fill the tube and keep it that way. Unless they are acrylic tanks and the bulkheads are below waterline thru the side and entire tube stay below waterline.