Is my fish tank overstocked?

I think you are at your limit. It is hard to say because I don't know what type of filtration you have. IF you have a good filter then I think you are fine. By good filter I mean it is rated for more gallons than your tank actually is ( example: Filter for 200 gallons used on 100 gallon tank). The beter filtration you have , the more you can get away with having the bioload(poop) higher. Good luck.