Purple fungus?

I got a pair of diamond gobies recently, both of which died within 1 1/2 weeks. The only odd thing that I noticed was that they both had some sort of purple smudge near their tail, is this some sort of fungus, or what?
 
I got no help. Never heard of a purple fungus. Maybe the purple smudge was just part of its coloration? I've never had one but aren't diamond gobies expert level fish with very special feeding requirements?
 
Back
Top