I keep seeing people diagnose velvet whenever a fish is covered in too many white spots to count. There's this popular saying: "If you can count the spots, it's ich. If you can't, it's velvet."
This is a myth, but it's so deeply ingrained in the hobby that people keep repeating it as fact.
In reality, a fish covered in hundreds of tiny white spots is much more likely to have a very severe case of ich than velvet.
Velvet primarily attacks the gills. Fish start breathing rapidly, stop eating, hang in water flow and can die very quickly, often before any visible skin changes appear. The idea that marine velvet looks like fine dust is largely a misconception carried over from FW velvet, which is a different parasite.
Ich, however, can go unnoticed for several cycles, since the fish usually keeps eating and acting mostly normal. By the time you notice something is wrong, the parasite may have multiplied enough to cover almost the entire fish in tiny white spots, making it look like fine dust. The number and size of spots also vary between species. This is why it's important to regularly observe your fish under white lights, as blue lights can hide early signs of infection.
Brooklynella can also cause visible skin changes, usually excess mucus, cloudy patches or peeling skin. These can sometimes be mistaken for ich/velvet too.
Does the difference matter?
Yes and no. Both ich and velvet can be treated with chelated copper at 2.25-2.5 ppm in a hospital tank. Brook, however, requires different treatment, as copper isn't effective against it.
Unlike ich, velvet can't be treated with hyposalinity (as it can survive in very very low salinity) or standard TTM. Ich management methods (such as UV) can't help with an active velvet outbreak.
If you suspect velvet, get all exposed fish into therapeutic copper of 2.25 ppm ASAP. Do not ramp up - copper takes time to work and velvet often kills fish before treatment can start working.
TLDR: Lots of spots doesn't mean velvet and no spots doesn't rule it out. Rapid breathing and how quickly the fish deteriorates are much more important clues.
If you suspect your fish might be sick, I recommend recording a 30s video under bright white lights (no blue) and posting it to Reef2Reef Fish Disease Treatment and Diagnosis forum for an accurate diagnosis and best course of treatment from fish disease experts. It's critical to start the correct treatment ASAP, as in some cases you may have less than 24h before fish start dying.
Some useful references:
