microbubbles,
I've never used cordon methylene blue and formalin in my opinion is much more harmful to fairy wrasses than helpful. I know a lot of folks who use formalin on other fish for treatment. But every treament I've seen with formalin on a sick fairy wrasse has been deadly.
My quarantine methods highly revolve around very low salt levels in the water which naturaly kill off the parasites if they are kept in that enviroment for a long time. I use maracyn two for torn fins, scale damage, popeye, low appetites, cuts, gashes...etc. If necessary I use low doses of copper. However, I tend to not buy fish that are sick in the first place if at all possible. Sometimes I come across hard to get fish that are sick and so I take the chance. But the best thing to do is buy from a good source and then your quarantine efforts are just standard and don't have to reach such drastic proportions.
I'm not an advocate of formalin at all for fairy wrasses. I think bigger fish like triggers and angels can react better to this but fairy wrasses just don't handle that stuff well IMO.
