Common Aquarium Problems Pt 4 Aggressive fish behavior can turn a peaceful tank into chaos overnight. When I first started, a neighbor gave me some mellow little swordtails—and I wanted to spice things up. I added a firemouth cichlid, not realizing how territorial it could be. By morning, the swordtails were gone, and I learned the hard way that not all fish play nice. These days, I use plants, caves, and hiding spots to keep the peace. A little research goes a long way in building a tank where