Just need a reducer and a small section of pipe to go in the union on the reef mat. Not sure what sizes you will need because the measurements I'm used to are bald eagles and big macs, but just a reducer to go from 32mm to 20mm.
Just be sure to measure the outside diameter of your drainpipe and the inside diameter of the union on the reef mat to get proper size.
Something Like this:
Slip x Slip. PVC Pipes for your plumbing needs. Use Schedule 80 Reducing Couplers to change plumbing size. More professional look than white Schedule 40 PVC. The golden rule of plumbing is that every job takes three trips to the store. While this can be frustrating locally it can be much worse...
www.bulkreefsupply.com