"Velvet disease, also called gold dust disease is a fish disease caused by the dinoflagellate parasites of the genera Oödinium, Amyloodinium or Crepidoodinium which gives the fish a dusty, slimy look. The disease often infects fishes in tropical aquaria."
- Above is an extract from wiki search.
Could it be velvet? Does he look kinda slimy?
Symptoms:
Scratchs against hard objects
*Fish is lethargic
*Loss of appetite and weight loss
*Rapid, labored breathing
*Fins clamped against body
*Fine yellow or rusty colored film on skin
*In advanced stages skin peels off
Initially the fish rub against hard objects trying to dislodge the parasites. As the disease progresses the fish becomes lethargic, fins are held close to the body, appetite is reduced and the fish loses weight. A key symptom is difficult breathing, resulting in rapid gilling.
The most telltale symptom is the appearance of a velvety film on the skin that looks like gold or rust colored dust. The film could be hard to spot, but can be easily seen by shining a beam of a torch on the fish in a dark room.
Just a thought?
If it is Velvet the treatemt is much the same as for Whitespot/
ICH.
Treatment :
*Raise water temperature
*Dim lights for several days
*Add aquarium salt
*Treat with copper sulphate for ten days (
I would advise not to dose your tank unless you really have to! - last resort)
*Remove carbon filtration during treatment
*Quarantine fish if possible
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Your guy is a goldfish so I'm not sure how he'd take to this kind of treatment buy - Hey, there's no guarantee it's velvet. Just a thought when you mentioned the slimy film on his head...