We now prefer using Maracyn as it's the same active ingredient(erythromycin). The price of API Erythromycin keeps going up multiple times a year. Maracyn has remained a better deal.
This is what we use here at the store and at Cory's fish room to treat every fish we come into contact with. For us we've never had problems using it with inverts, scaleless fish, and plants. We've never had our beneficial bacteria die from this medication, but as with all medications take precautions in case your cycle does crash.
Commonly used to treat:
For best results remove activate carbon. For each 10 gallons of water empty on packet directly into aquarium (For 850gm jar, add one scoop [1tsp] per 20 gallons), repeat dose after 24 hours then change 25% of the aquarium water. Repeat treatment a second time for a total of 4 doses, then make a 25% water change. Treatment may be repeated if necessary. Four doses required for full course treatment.
Treats 100 Gallons. 10 Packets, each treating 10 gallons.