Velvet is caused by a dinoflagellate, classified by some as an alga because it carries Chlorophyll.
There are two main species, Oodinium limmneticum and Oodinium pillularis.
The free swimming stage of Velvet settles on the skin and gills of the fish,
adhering at first by its long flagellum, later putting out pseudopodia, (similar to fingers) that penetrate the skin and give it a very firm grip.
Velvet has a similar life cycle to White spot, feeding and becoming a cyst it produces upward of 200 young before dropping off.
The appearance is similar to gold or brown dust over the body and fins which at times may appear to move.
Symptoms are similar to White spot, glancing off rocks etc.
As Velvet is highly contagious it is important to eradicate this problem as soon as possible.
Treatment is aimed at the free swimming stage and there are good cures available from your local store.
Copper sulphate can be used at a concentration of 0.2 mg per litre or 0.2 ppm.
This should be repeated after 3 days to ensure eradication.
Fungus (Saprolegnia or Achyla
FungI are in fact colourless plants, they are very diverse in in form and have not been studied that much in fish.
Saprolegnia and Achyla are genera of fungI that attack weakened and injured fish, usually settling on damaged skin or gills, they will also attack the eggs of fish.
The spores enter the aquarium from the air so there is no way to stop possible outbreaks.
Threads of fungus spread under the skin forming a web like structure and eventually produce tufts of external hyphae that may be large enough to look like cotton wool.
These form spores that become free swimming and can then go on to infect other fish that are weak or injured.
It is important that you remove any fish you suspect immediately to a hospital tank for treatment
Symptoms of fungus are a grey or whitish growth in the skin of the fish, often associated with visible damage.
Treatment can be with Malachite green in a separate tank for 30 seconds at a strength of 60 mg per litre, repeat treatments may be necessary.
The fungus is stained by the Malachite and usually drops off within a few hours.
Keep the fish under observation for 2 or 3 days as fungus can sometimes regrow.