I would dose with Prime (or something similar) to help reduce the amount of damage your fish are incurring from the current ammonia. Then I'd look for another healthy tank at a friend's house or a local fish store to see if I could get a bunch of their filter media to seed my tank. Bacteria is what you need to lower ammonia, not more fish. Bacteria, in the proper proportions, will also be what you need to keep your nitrites at zero. You'll need to do water changes to remove nitrates. Plants can help with nitrates as well, but they don't replace water changes by themselves.
If you can't find another healthy tank that's been up and running for a long time, look into purchasing some Tetra SafeStart and dump the entire bottle into your tank.