Hi Shepo
OK, do a 50% water change with a thorough gravel clean, now this is what i used and it worked for me and to this date the fish are still doing very well and getting stronger every day. Get a general tonic with Acriflavine & Methylene blue and follow the dosage guide, I only added 10ml to my 180lt but it should have been 27ml, i let this run in the tank for 4 days then i added carbon to the second filter to slowly run the med off, aslo increase the airation .
The General Tonic i used was by NTLabs and it is plant and filter safe but it does send your water lime green. The general tonic will remove the virulent pathogens that will be in the tank water and claims to aid the Gills of the fish, now this worked for me but in the first few days of treatment i still lost a couple of fish but after that the fish deaths stopped.
It is up to you if you do this or not .
Good luck mate.