A 2 gallon tank is really too small for much of anything. So is a 5 gallon hex. A hex tank, because of the surface to water ratio, should be stocked as if its only half the size. Your 5 gallon hex provides the same space ratio as a 2.5 gallon square tank. You wonder why the water quality went bad. You'll need a filter and you'll need the tank cycled if you want to add any other fish to the betta tank. You really should have a filter and cycled tank for bettas too. They breath air, yes, so they don't need the oxygen a filter provides. But they do suffer from the ammonia levels that build up daily before you change the water. If anything, you could add one platy to the betta tank.