Probably on account that it's competing for PO4 (zeo will pull down PO4 as well). Apparently in some cases a slight amount of PO4 (or more specifically the ratio of NO3 to PO4) is more important than the actual numbers themselves (and both are used up organically) so maybe running both simply depletes it too much perhaps. I'm just guessing. It would be a good question to ask over at zeovit.com to see what spin they will have on that. If you do happen to do that please post back here what they say.
I hope I didn't put you off by calling it old school - not my intent. Like I said, it works great, it's just an idea that's been around a while (I was running both ... in 2002.
). I don't anymore though, I use my CO2 on my FW planted tank and just dose the reef tanks (using a ghetto but automated setup). It's more economical for me this way since I run multiple reef tanks.
One nice thing about a kalk reactor is that the output can be split to different tanks so it lends itself well to the many-tank-syndrome people. Calcium reactors cannot.