It uses erythromycin which typically affects gram negative bacteria ribosomes, Nitrospira and somonas have a strong resistance to it in my experience (more like gram + structure) and I havent had to worry about using it. Plus they usually make a large biofilm that is hard to penetrate. You should be fine, I didnt see any issues when I was treating my goldfish for almost 3 weeks on it. Others may have had different experiences tho.
But you may want to try H2O2 first. Remove filter and use some powerheads to let it flow for a bit(15-20), add 2TBsp for each 10gal . then do a 50% change
Last edited by Mirkinator; 04-12-2014 at 07:02 PM.