Well powerhead wise i would say you would need more than two, pending the two you have arent super huge pumps (which i doubt or you wouldnt have flow issues). I would say finding a few decent semi high rated pumps would be your best bet, 3 or 4 maxi jet 1200's would be a good start for a 75 more if you decide on more corals down the road, your light are probably fine the system is just going through a shock right now,I'm guessing you had either lower par lights like pc's before or your bulbs were really old. Are you running the lights for 8 or more hours? If yes thats probably part of the problem too, if it's the first time youve had halides on the tank you should cut back your lighting time to give the tank and corals time to get use to the lighting change. I dont have any idea where you could find a used return pump, you could always try fosters and smith for a new one