Eels

Eels can be a very cool centerpiece of an aquarium. They aren't commonly seen in pet stores, and this is for a reason. Eels can't be put in most community tanks without a few requirements being met beforehand. Most eels are semi-aggressive or aggressive. This means that since they lurk at the bottom of tanks, it isn't a good idea to have many bottom dwelling fish that can't handle some harassment. Also, eels like to be hidden most of the time. So it is important to have plenty of hiding spots and plants to make them feel safe. The last factor that should be looked at before getting one is the substrate that you have. Eels stay at the bottom of the tank primarily, which entails their stomachs rub against the substrate a lot. It is important that you have a sandlike substrate so they do not injure themselves. If you have sharp gravel, it will cause scrapes on the eels stomach.