Dwdenny,
Well I have a quiet one 6000 doing exactly what you are planning on using a 5000 for, and mine seems to work fine. I definetly have a heat "issue" in my tank, but I suspect it is the Mag 12 and mag 9.5 in my sump , and not so much the quiet one. One thing to note is that the threads on the pump seem to be a slightly different pitch that the standard NPT. The fittings that came with the pump fit perfect, but I used a threaded PVC end, and it would thread on, but it would bind up when I tried to take it off. Be careful as I acutally broke a 4000 trying to take a fitting off. (acutally cracked the impeller magnet!). As far as quiet, it is reasonably good, makes some vibrating kinda noise, but you might be able to get that to go away by mechanically isolating it better, MUCH quieter that the Little Giant pump I originally put in it's place.
Just my 2 cents....
Jack