If visible physical symptoms do manifest:
Velvet may initially start out looking just like ich, with salt or sugar-like “sprinkles” visible mostly on the fins.
Within days or sometimes just hours, these tiny white dots will spread all over the fish’s body, covering it in “dust.” This dust may look grey-gold colored if viewed at the right angle and under the right spectrum of light. For this reason, it may be difficult to see velvet on a yellow or light colored fish (look from an angle, not directly from the side). However, sometimes a fish’s body will look “dirty” or show “dark areas” just before velvet appears.
There are two ways to differentiate velvet from ich:
Velvet trophonts (and the correlating dots) are much smaller than ich. They range in size from 10-80 micrometers in diameter. They are also perfectly round. Ich trophonts are more oval shaped and range in size from 48 x 27 to 452 x 360 micrometers. Both start off small but then grow in size before dropping off. It has been said that velvet makes a fish look like it has been dusted with a fine powder, whereas ich is more like salt grains.
If you can count the number of white dots on your fish, then you are probably dealing with ich. If they are too numerous to count, it is most likely velvet.