em erythromycin

I have 6 glofish, 2 glofish tiger barbs, 3 rosy barbs, 8 zebra danios, 3 panda garras, and 1 Amano shrimp in a 20 gallon tank. Parameters are 7.4 ph, ammonia 0, nitrates 10-20. Feeding is once a day with fish flakes and about once a week I drop in an algae tab and some sinking shrimp pellets...