I haven't run into true velvet (Amyloodinium) in my own tanks for a number of years. Early on, 40+ years ago, when I didn't quarantine with copper, I would lose a lot of fish to this - they would start breathing fast, stop eating and then begin dying over a period of just a few days.
We see a lot of people you mistake advanced cases of ich with velvet. Typically, if you can see individual white spots, it is ich, not velvet. That's a fine point though, as copper works for both of these issues.
Here is some text that I wrote about this disease:
Amyloodinium, (a.k.a. marine velvet disease)
Cause
Commonly known as “marine velvet” in hobby parlance,
Amyloodinium is caused by a dinoflagellate protozoan that can create severe epidemics in aquariums. Furthermore, it can infect fishes that are normally more resistant to other marine protozoan diseases (e.g.,
Cryptocaryon), such as sharks, rays, and eels.
Symptoms
The life cycle of
Amyloodinium is very similar to that of
Cryptocaryon, as are the possible treatments available, but it has less distinctive early symptoms and can cause fish mortalities much sooner than other protozoan infections—sometimes within 24 hours of the onset of obvious symptoms. This disease begins as an infection of the fish’s gills, and only in advanced cases does it spread to the skin, giving it a “velvety” look. Symptoms include rapid breathing (greater than 140 beats per minute) and hovering in the current from pumps.
Beginning aquarists often miss the first symptoms and commonly report, “All my fish suddenly died, but the invertebrates are all fine.” Since invertebrates are typically more sensitive to water-quality issues than fish are, the fact that the fish suddenly died but the invertebrates were unharmed means that water-quality problems can be ruled out. That leaves a fish disease, and
Amyloodinium can often be diagnosed without even needing to perform a necropsy on the fish due to the rapidity of the fish loss!
Diagnosis
The key to early diagnosis of
Amyloodinium is to monitor the fish’s gill health by taking regular fish respiration rates. This is a simple matter of counting the number of gill beats in one minute for a representative fish in the aquarium and then rechecking the respiration rate every few days to watch for any elevation in that rate.
Newly acquired fish that are not being treated prophylactically should have their respiration rate checked daily, as these fish are the ones at greatest risk of developing this disease. The actual respiration rate is not that important, it is a rise in the rate that must be monitored for.
Different species of fish will respire at different rates. Smaller fish breathe faster than large ones, and fish in warmer water will respire faster as well. Typically, tropical fish will respire between 60 and 120 gill beats per minute. If you can’t view the fish for a full minute, you can try counting for 15 seconds and multiplying the result by four.
Knowing your fish’s normal baseline respiration rate is vital; any rise in that rate above 30% (and not attributable to something else, such as the fish being chased by a tankmate) should be viewed as a possible symptom of this disease.
Treatment
Treatments for
Amyloodinium cannot be performed with invertebrates present, yet the entire tank usually needs to be treated in order to eradicate it. Copper sulfate at 0.20 ppm for 14 days is one often-used cure. Chloroquine at 8 to 15 ppm as a 30-day static bath is another treatment that has been used with good success. Amine-based copper medications for 30 days is also effective.
Performing a five-minute freshwater dip can buy some time in order to develop a full treatment. Hydrogen peroxide dips at 75 to 100 ppm and moving the fish to a sterile aquarium has been used in aquaculture. Lowering the tank’s temperature is rarely effective. Likewise, hyposalinity treatments (sometimes recommended for
Cryptocaryon treatments) will not work for
Amyloodinium.
