The drum moves out from the feeder so you have to make sure there's an opening for the food to drop. I just checked my order. I had mine made 3 1/4 inches wide and 1 3/4 inches from the inside edge of the tank.Thank you! Can the slot be made any smaller or is that about what it needs to be for the feeder? I haven't ordered the feeder yet so I don't know what it needs.