Did they show up rapidly or just a few here or there every couple days? Either way if it's ich or velvet copper will take care of both, get all fish out of dt, into a qt, and treat all fish with copper. I use coppersafe and api test kit to get to therapeutic levels of copper. (1.5 to 2) also never use copper tank for anything else but copper....same with all equipment . ALSO NOTE TO KEEP DT FALLOW FOR 76 DAYS. This will ensure that velvet dies off and is not in the dt when u move the fish back. Treat with copper for a minimum of 30 days. Get an seachem ammonia badge to monitor ammonia. You can buy live bacteria to start the seed of bacteria to help with ammonia. DO NOT PUT ANY ROCKS OR SAND in qt with copper as it will absorb the copper and drop the level in the water column. You can use a sponge for the bacteria to live on to go through nitrification cycle.