There are small sharks that can live in a small aquarium like the perry's latern shark adult size less than 7 inches, and some bamboo sharks aroung 18 inches as adults that can co exist. I would study the species you choose completely, before stocking the tank with it. Most sharks need higher salinity 1.025 to sastain healthy. Other fish can live as well it's just things like this you should know before adding a shark to your tank. Again, very little species will be acceptable so the info will be hard to find due to bias and conciencious aquarists.