I used a reactor for years and they're all about flow. A reactor of this sort really cuts down on the flow from a canister. If I neglected filter maintenance I invariably paid the price with stunted growth (check out my journal and you can find a few pics showing this).
I think the issue is flow in the tank. I don't think a lone XP3 is going to cut it, especially with a big PVC reactor, as the only source of water movement in a 75 gallon tank that has or will have a lot of plant mass. You're going to need a filter that moves more water, a second filter or a circulation pump or two such as a koralia.
If your dropchecker is on the opposite end of the tank from the filter output try moving it closer and see if there's any difference.
Last edited by Jeff5614; 09-14-2012 at 03:21 AM.
Reason: spelling correction