I converted a wet/dry into a fuge. It was easy to do.
First, I drilled a 1.25 inch hole near the top of the wet/dry I was converting. Then I installed a bulkhead fitting in that hole. That bulkhead is like a spigot sticking out of the top/end of the converted wet/dry tank, and it becomes the return which spills back into the collection area in the working wet/dry (where the pumps collect water to send the water back to the tank).
I have two Rio 3100 pumps running the wet/dry. I put a splitter with a valve on one of the return lines. The valve allows me to control how much water goes to the fuge and how much continues up to the old return in the tank. I've been sending about 20% of the flow from one 3100 to the fuge. The line that takes the water to the fuge dumps the water in the far end - to maximize/ensure complete circulation, though the current is very slow. The water gradually moves through the fuge and then spills out the bulkhead into the collection area in the old/active wet/dry.
In order for this to work the bulkhead had to clear the top of the old/working wet/dry filter, so we made a stand for the fuge that raises it about 3.5 inches - then we just pushed the two tanks/wet/drys together. The threaded portion of the bulkhead sticks out over the edge of the collection area about 2 inches, and that's plenty to ensure that all the water goes where it's supposed to go.
