I have a 40 gallon tank and have 4 Clown Loaches which share the bottom of the tank with 5 Albino Corys. They get on absolutely fine and have all been in the tank for about 6/7 years. The Congo Tetras swim in the central area of the tank, and I disagree that you need a larger tank. I have 4 Congo Tetra males that are about 5 years old, and they are such beautiful fish I would not wish to be without them. They all seem content, and the fact they are the age they are seem so show that all is well in my tank. However that may also be because I do weekly 40% water changes, including cleaning the filter media each week.