It depends. I've seen it work over 100' and not work with just a wall between and the WXM and the Vortech (maybe 8'.)
Part of what contributes to it not working is the building material in the house. If you are working in an older home with sheet rock, then don't expect much. If you are in a newer home with dry-wall, then you'll probably be OK. Also if you have several other Zigbee type devices around your aquarium, well that might be problematic too.
Pair them right next to each-other for the best results. Hope it works out for you.