White crust in lips of Platy

Question
I have a 30 gal tank that has been up and running for about a month maybe a bit more.  I use some good bacteria via a stable tank and my ammonia spike was very brief and low.  I'm fairly sure the cycle has completed yet I never saw a nitrate spike. I do 20-25% water changes every other week. I have had two Platys one red wag and the other a Marigold both have been very happy.  I went to get them some friends about 5 days ago and everything was going good I acclimated them for 15 min then did added a lil tank water to the bag waited another 10 min and moved them to the tank. About 1 1/2 days later one of the sunburst Platys started darting around and then I noticed white crust on his lip, I looking on line thought I found the answer but it was too late the next morning he was dead.  I assumed it was just stress from the trip home and chocked it up to him being a weak fish, but then the next day my Marigold one of my original fish that was doing very well had the white crust. I'm starting to question the whole stress thing.  Before I could get to the pet store literally the next morning my marigold was dead. UGH!


So far the rest of my Platys look good but im worried there is something with my water.  I test it every day.
Nitrates 0-10
Nitrites 0
Hardness 50
Total ALK 120
pH 6.7ish
I know its alot of info but im at a loss and wanted to give as much as I can.

Answer
I would start with a 25% water change and then I would add aquarium salt.  You could treat them with General Cure which is a wide spectrum medication.  I am not sure what the white crust would be other then a fungus... but that should not kill them so quickly...so that leads me to thing of something caustic and the crust would be blisters... not sure though... keep me posted... dave