“Velvet dinospores can remain infective for up to 15 days, whereas with ich theronts it’s only 48 hours. This is because velvet tomonts and dinospores are both capable of using photosynthesis as a means of obtaining nutrients (remember it is a dinoflagellate).” - HumbleFishNo real sensitivity but I got a bunch of fish in a 20 gallon and its totally too small and unnatural. They need to get back into the DT asap. Also, while the lights are usually off, I believe that marine velvet is not photosynthetic. Freshwater velvet is.
https://www.reef2reef.com/threads/velvet-amyloodinium-ocellatum.217570/
Keep a sponge filter or air stone running if you can and do water changes daily (predosing the water to the desired level).