platy dies from parasite?

Question
I have three neon tetras that are abuout a year old in a 10 gallon tank (changed from 5 gal about 3 mos ago).  About 2 weeks ago I removed my pleco that got too big and added two upside down cats and a mickey mouse platy. Today I noticed a growth or scratch on the side of the platy.  The scales were lifted with some amount of fuz and a red line (blood?) around the affected area. The spot was located half way between the tail and dorsal making the spine bend a bit and fish swim uneasily.

I took him in to the store where I bought him and they said it looks like a parasite.  The other fish looked fine, but they suggested I treat with a blue tablet (forgot name and box is with tank at work).  

Do you think they have the diagnosis right?  Must mean I got it from their fish, so should I buy fish from them again? I did treat the tank today with the tablet, but is there more I should do? The platy was so bad in just one day of dealing with this that I had the pet store "take care" of him!  He ended up on his side and not swimming much in his quarantine bowl.  

Answer
Hi Kim;

It sounds like an injury that became infected to me. A parasite isn't going to cause a bend in the spine but an abcess might. If the fish was bitten by another or injured in some other way it can develop that kind of sore. It may have happened before you ever got her and just got worse over time after you brought her home.

The medicine is probably not needed because I really don't think the other fish are in danger. I'm really not a fan of using medicines to prevent infection that isn't there anyway. It can make disease-causing organisms to become "immune" to the medicines and then they don't work if you ever need them for sure. Look at the fish carefully to see if any of them look sick or abnormal in any way. If they don't, I wouldn't treat them.

I hope all goes well...

At Your Service;
Chris Robbins