I have some bloodfin tetras and zebra danios, and some cherry barbs. I know the tetras and danios are suppose to be middle to upper level and the barbs more middle but all of these guys hang near the bottom I'd say 85-90% of time. The tank is heavily planted with floating plants but still plenty of swimming room in the middle and upper levels. What causes them to act like this? They're cories,and mollies in the tank also and sometimes the bottom looks very crowded as the cories spend most of their time down there and the mollies are everywhere. It's a 55 gallon. These fish have been in the tank for over a year and have always preferred the lower half of tank.