maybe this will help,
Velvet (Piscinoodinium pillulare) is a flagellate parasite with chloroplasts, while ich is a ciliate, both have similar life cycles, but unlike ich, velvet can use its chloroplasts during dormancy to survive (which is where light can help), but in a full blown infestation, the parasites get their nutrients from the host and are NOT dependent on their chloroplasts.
This is an area of some controversy as some old school methods claim light and salt will treat and cure velvet, this has been proven false along with the assumption that this is a parasitic algae (due to having chloroplasts), this is not true. This is a flagellate protozoan.
This parasite is more resistant to higher salt levels than ich (Ichthyophthirius multifilis) and uses the chloroplast to produce nutrients (via chlorophyll and light) when a host is not available or to supplement nutrients derived from the host. It should also be noted that Marine Oodinium parasites do NOT have chloroplasts
What Are These Parasites?
Piscinoodinium pillulare are flagellate protozoa, single-celled organisms that move around by thrashing their whip like flagella. There are many species of flagellates that are part of the normal intestinal fauna of fishes, and many more kinds of free-living flagellates make a living in the mucus of fish gills and skin, without attaching themselves or causing trouble, but even some ordinarily harmless ones can become pathogenic in stressed hosts. Piscinoodinium pillulare is not harmless; it puts down a root like extension and can burrow into the skin or gill tissues. Giardia lamblia is a similar flagellate protozoa in humans that can cause severe acute diarrhea which may lead to a chronic diarrhea and nutritional disorders.
Identification;
Look for fish that scratch or ‘flash’ their bellies in attempts to scrape their gills against stones or gravel.
Rapid respiration or fish that hide or sink to the bottom and clamp their fins, in classic symptoms of malaise. Velvet moves fast, faster than Ich. If you're unwary, you may not realize the fish is being attacked by Velvet until it begins to lose its glossy shine and seems to have patches of yellowish to golden-brown or rusty-colored varnish (which is where the term ‘Velvet’ came from). If you turn out all aquarium and room lights and point a flashlight at the fish, this will make Velvet more visible.
Velvet can also settle out in the gills, where it will send down a root like extension into the gill lamella and dissolves cells then absorbs their contents. This causes extreme gill itch and swelling. In really serious infections, the gill cover may also become swollen. Fish can be killed by Velvet in a few days, either directly from suffocation in the Gills or from secondary bacterial infections.
Treatment:
Copper Sulfate is one of the effective treaments because of this, as it kills both dormant (destrying the chloroplasts) and kills the free swimming stage as well.
Methylene Blue (this should be buffered or used in a hospital tank) is also very effective and has the added bonus of being a hemoglobin transfer agent for the blood just when the fish need it most.
Acriflavin, and to a slightly lesser extent, malachite green and formalin are also useful
This is why the Medicated Wonder Shells are one of the best treatments for FW velvet.
These medicated blocks contained buffered copper and methylene blue, two of the better treatments for this infestation along with malachite green and Acriflavin. These blocks also add electrolytes and calcium, both essential to good healing
Back to lighting, this may help with low infestations just like salt does with ich in low infestations, but in a full blown infestation, just like salt it is overrated.
I have witnessed this first hand in my Aquarium maintenance business, even with lights off!
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Freshwater Velvet - Piscinoodinium Pillulare (also known as oodinium)
