Well, first off, your fish do not 'fit' the size of the tank. Normally, you research what their needs are and you see if you can provide them. This is the only way for the fish to lead a relatively happy life in your setup.
I would strongly advice against more plants, since they take away all swimming space. I tried the same thing, and I would end up with all my fish crammed into a corner of my tank, where there were no plants. Their coloration got worse and everything, so it's not good for them that they aren't able to swim more then an inch before having to make a u-turn.
Your 20g is still overstocked, although some fish passed away. You could pull it off if you have extra filtration (more then that what came with the tank). But I still would advice you to return some fish, and definetly your bala shark, and tiger barbs.