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One of my male Platys has lost weight. All the others (5 in total), are thriving (so are the tetras and loaches). His head/face look gaunt but his colour and fins are good. Any thoughts appreciated.

It can mean a lot of things, from old age to disease to just not getting enough to eat.

Last summer, one of my male platies lost a lot of weight, but his color and behavior were fine. He's never gained the weight back, but a year later, he's still going strong. I think he's nearly 2-1/2 years old now -- that's getting up there for a platy -- so maybe it's age related.

I had another fish, a rosy barb, who also got really skinny. I isolated her to observe her, and quickly discovered that once she was by herself -- i.e., got all the fish flakes to herself -- she put the weight right back on. She was just getting out-competed at feeding time by the zebra danios in the tank.

There are diseases -- parasites, etc. -- that can cause weight loss, too -- maybe other members will have suggestions for you in that regard.

Loss of weight can mean fish TB, a contageous malady that will spread and infect all fish in the tank...

Advanced fish TB will reveal skin ulcers and a general wasting, evidenced by loss of appetite.

--garyroland.

it could be an internal parasite those usally cause fish to lose wieght and by the way how do you tell!

Yep it might be an internal illness like tuberculosis.
I think that's garyroland is talking about.

For info on diagnosing and treating it go to

www.fishprofiles.com/interactive/disease/tb.asp

Good luck to you and your platy.