it really would well worth the little amount of time (probably realistically a few hours) to turn off your feed pump, disconnect plumbing, drain a few gallons out of your sump and set the sump up outside of the tank. as it is, to run externally you are going to have to drill that sump tank anyways because the return lines of an external skimmer have to get back into your sump somehow....and if you cant have it high enough youll have to drill it. u remember how my external is, it sits above the sump. its just a fact that you should have thought this threw a little more to be honest. wen all have issues here and there but this one you DO NOT want to skimp on if you are planning to have a sps dominant system. but dont take my advice for it, i dont know much about sps