I think you'd be fine. I doubt you'd get a nitrite spike. I can only speak from personal experience. I just set up a 240 using 2 established XP4's. I also pulled some bioballs out of my established wet/dry and used them on the new tank. I did this on 1/13/08 and I haven't had any problems. No ammonia spike, no nitrite spike and now I seem to have Nitrate readings, so I think I'm in the clear.
Water Parameters:
1/13/08 Ammonia: 0 Nitrites: 0 Nitrates: 0
1/14/08 Ammonia: 0 Nitrites: 0 Nitrates: 0
1/15/08 Ammonia: 0 Nitrites: 0 Nitrates: 0
1/16/08 Ammonia: 0 Nitrites: 0 Nitrates: 5
I would recommend throwing some media from your FX5 into the sump with your bio-balls to get some beneficial bacteria in there jumpstarted.
The way I understand it, you are simply adding surface area for the beneficial bacteria to grow on. So adding the filter to an established tank is probably not much different from moving established filters to a new tank. My new 240G has tons of surface area for new bacteria to grow on (56 square feet of tank walls, new rocks etc) and I have not had a Nitrite spike whatsoever. My theory might be flawed, but I think it makes sense. I will let you know if anything changes over the next few days. Test your water and make sure things stay within normal limits.
If you want to see my thread it's at:
http://www.monsterfishkeepers.com/forums/showthread.php?t=204684
Let us know how it works out. I'm curious if my experience is out of the ordinary. People seem to have different opinions on this topic.
