15 fish is not too many for a 20 G, its the kind of fish you also have to consider. I have 6 female bettas, 8 false julii corycats, 3 albino corydoras, 2 upside down catfish, no idea how many ghost shrimp and 2 or 3 nerite snails. My tank is 16 gallons.
My tank is planted and the water tests are always perfect. The tank is not as crowded as you think. They all have plenty of room. The corydoras though seem crowded but they enjoy shoaling and there is enough space during the day I can't find half of them until feeding time.
I've about hit my limit. I may add 2 more female bettas in the future.