Like any antibiotic there is a risk that it could kill the bacteria in your filter and cause an amonia and nitrite spike but the risk is proportional to the load (stocking rate of fish) on the tank so watch for that. I once lost a tank of fish treating with Erethromycin but have got away with it on other occasions. I have used furan2 and had no problems, but it is not cheap. You use one cap/40 litres, remove what you can and treat again next day then remove what you can again.