That's ridiculous. The right
LEDs are the most efficient and powerful lighting you can put over a tank to date. They leave ancient metal halides in the dust. Whoever told you that was clearly misinformed.
Your frag tank would only have to be above the common sump. The position of it to that of the DT is not important, unless you are draining the dt into it, and then it into the sump, which I would not recommend.
Something like this is what I would do-
Though you have a few options for what to put inside the tank-