Hello,
I Have a 20 litre tropical fish tank with a heater,filter and light. I currently have a male betta and 5 black neon tetras.
What other additions could I add to my tank without it becoming overcrowded
Boy you are pretty much stocked. Snails. You might be able to do some cherry shrimp. That is a tiny tank. I would work on getting yourself a 10 gallon for those fish and ditch the 5 gallon.