I would add more LR. If you have a good LFS then they probably have cured LR and you should add a little at a time until you get to around 30lbs. The LR will act as a biological filter for your system. There's no need for expensive equipment if you have enough LR (1.5 to 2 lbs LR per gallon suggested).
I do think that you are overstocked, but there are remedies. Like I said, you should take some of the fish back to your LFS. A 20g is reasonable, but you need to have a plan. I have 2x 20gallon tanks and I have learned through trial and error that you need a goal for a tank that size. Plan on having at most 3 fish, and they all must be compatable. There isn't much room for a bullied fish to run.
There are some good websites that can offer lots of info about what kinds of fish can live in a 20g. If you want more info about these sites or my tanks I will be happy to share what (limited) information I have.