Erythromycin is a ingredient. It is found in the products Mardel Maracyn and API E.M. Erythromycin.
I could have sworn I read that ChemiClean was just Erythromycin, but looking it up again, it seems it is not and in fact it states it does NOT contain Erythromycin, but the actual ingredients still seem to be kept under wraps/a secret (even under their MSDS). Either way, it still works on Cyanobacteria and I haven't heard any harmful reports toward fish, inverts or plants.
I haven't personally used Erythromycin first hand as I've never had the need to use it, but I have read up on it. Erythromycin is a antibiotic and mainly targets gram-positive bacteria (it does effect some, not many, gram-negative bacteria). I've read from many that erythromycin had negatively impacted their nitrifying bacteria, causing a mini-cycle.
Upon researching aerobic nitrifying autotrophic bacteria (nitrosomona and nitrospira in particular) I get conflicting information whether they are gram-positive or gram-negative. I have seen some pretty knowledgeable sites (even some knowledgeable fish keepers on various forums) say they are gram-positive and some say they are gram-negative. If any one knows for sure what they are, can you please inform me and possibly link a source (preferably a proper scientific report). But to the best of my knowledge, it makes more sense that the beneficial nitrifying bacteria in our filters are gram-positive.
Not all antibiotics target the same type of bacteria though, so that can explain why some don't seem to harm the beneficial bacteria as much or at all. Most meds that do end up harming the beneficial bacteria, apparently only harm some of the beneficial bacteria (I've read that biofilm acts as a shield/protection), causing a slight mini cycle, but enough survives the treatment and the beneficial bacteria colony recovers/repopulates fairly quickly.