Tank is about six months old Waterbox 75 litre. Ph 8.0 NH3 0.001 Calcium 489 Mag 1140 Phos 0.00. Always do weekly water change of 10%. I had this about a month ago and a dose of Chemiclean got rid of it in a few days butthis time, dosing doesn't seem to get rid of it. Only two Clownfish in the tank who are fed every second day and only what they can eat in about two minutes. I have just added four zombies and four peppermint shrimp as I thought this might help.