I have a 18 gallon tank, and I had 2 guppies (one male and one female), 2 leopard danios, 2 aquatic frogs, and a plecostemus. (and a black mystery snail). I had no problems, really.. after a coule nips to the danios fins, he left them alone. The only problem, was after the fact, they all got ick, and my 2 guppies died, and my betta got it pretty bad. He's just fully recovered recently, and, I plan to put him back in the tank as soon as the cycle is finished.