Fire eels can get as large as 36"-40" long ( 92-102 cm). Growth rate will be quick if the eel is fed properly. Stick to meat foods ( i.e. shrimps, worms, small fish) and don't even think about prepared foods or sticks. Most likely they won't eat them. With an eel that small ( 6-8 cm) best foods for them will be frozen bloodworms, live black worms ( or frozen if live is not available), thin strips of fish ( no wider than the diameter of the eel) or small shrimps. If you can get livebearer fish fry ( i.e. Mollies, platies, Guppies, etc.) those too will be good foods for the eel.
Tank capacity ( liters or gallons) is not really as important as tank dimensions. A tall tank may hold more gallons/ liters but still not be good for a fish that needs lots of swimming space left to right. So by the time the eel gets to 60 cm (+/-) you'll want to upgrade the tank to one that is at least 182-244 cm long so that the eel has plenty of space to "swim around" in and not feel cramped.
Tank mates for eels can be a little tricky. You want to stay away from species that are long and slender because that makes them easier for the eel to swallow. You'll want to stay away from bottom dwelling species since they will be competitors with the eel for space. There are a number of larger species of Tetras such as Bleeding hearts, Columbian Blue and red Tetras, Black skirt Tetras or Congo Tetras that will get more round than long so not really an easy prey for the eel and will stay more in the mid to upper sections of the water column so not really interfering with the eel's space. Some of the larger barb species are another choice but again, you want to stay with species that are not bottom dwellers. As for cichlids, there are the Severums, Angelfish, Uaru and Discus ( think hard if you go with Discus as they will require a lot of work) that are more round than long and slender. Just make sure you start with small ones so they don't out compete the eel for food.
So there are some suggestions for you. Hope this helps.
