This size tank is fine. Plant the tank well with some floating plants like Water lettuce, Pennywort, Water sprite and add a piece of floating driftwood. I got pieces from the local river areas. I attached Java fern to the wood with long strands of cotton sewing thread. Use black thread, it kind of blends in with the wood. The cotton thread will eventually dissolve in the tank water. By then, the roots of the fern will be established in the crevices of the wood. All these things will make good hiding places for a bullied fish and make all the fish more comfortable in the tank.
"Fear not, my young apprentice. Just change the tank water."