what is considered an over crowded tank? How do you know if your tank is over crowded? I have a 27 gallon tank fully equipped with a heater keeping the water at 78*F, a filter that cycles about 500 gallons per hour, an LED light that mimics the moon phases, and several real plants scattered around the tank. In total for fish I have a female betta (2 inches long) who gets along really well with other fish, 3 neon tetras (all under 1 inch in length), 2 Blackfin tetras (about 1.5 inches in length), 3 Fancy guppies (all about 0.5 inches in length), 2 Lyrtails (each 1 inch long), 1 butterfly plecostomus (almost 3 inches in length), 2 other really tiny fish (not even 0.5 inches in length) and 3 snails about the size of a quarter. (total = 14 fish and 3 snails) All the fish get along really well. Most of the fish I have are schooling fish and will swim together in groups. My betta will sometimes chase my other fish but is never aggressive. She has been in the tank with the Neon Tetras for over 9 months now. I have minimal algae growth (none on the glass, some on the structures in the tank but the pleco and snails keep up with it), no wast problems and all my nitrate, nitrite, and ammonia levels are 0, my pH is 7.0 and my water is completely clear with no issues.