You are golden, don't worry about the DT. Just pretend you don't have sump anymore
The issue would be is the sump-water is 'stagnant'. And if its anything like my sump, its probably dirty as sin.
You might want to do a water change of whats in the sump out, or at least dilute it, before recirculating it up into the tank again. Unless you also have powerheads in the sump or something keeping it moving.