The best absorbed seem to be the nitrofuran antibiotics such as nifurpirinol and nitrofurazone. The absorption of other antibiotics is generally poor, which is why such treatments often fail against advanced and systemic bacterial infections. For longer-term bath treatments, this will severely restrict the numbers of fish that can be treated if ammonia levels are to be kept to an acceptable level without biological filtration. It is important to test and monitor water quality during prolonged treatments. It is important to use the purest possible drugs to avoid any side effects from carrier compounds.
Many antibiotics chelate divalent cations such as magnesium and calcium, causing their inactivation.
This is particularly the case with all tetracyclines and quinolones such as oxolinic acid and enrofloxacin. Degraded tetracycline can be harmful to humans so avoid contact by wearing gloves.