I have just lost 9 fish to ich and after much research this is my advice.
Get a QT tank. All not some but all fish in your DT must be assumed to carry the parasite so remove all of them.
Ich is most likely in the gills and mouths of those with no spots. They all must be removed from the DT if corals , live rock, inverts present. Your DT must set fishless but do all the maintanance for 45 days. The ich will literally go though its life cycle without hosts and starve out in that time.
Ich once it detaches from the host will sink to the substrate and is non infectious for a short time as it multiplies. It then becomes free swimming in the tank and finds a host. Once a host is infected it may time some time to see outward signs. The whole vicious cycle then begins again.
I won't try and tell you the best treatment in the QT. Some say hyposalinity bringing the numbers down in the QT to 1.10-1.15 for a specific amount of time. Others say Cupramine or ParaGaurd works well.
Others say only Cupramine or hypo works at all. Cupramine and hypo are tried and true and for specific treatment of ich that is most likely your best choices.