That may not be hard water buildup - if it's truly detergent residue, you may just need to cut down on it a lot. If you're filling the dispenser any more than half-way, that's probably too much unless your water is extremely hard.
I don't notice any residue buildup at all using powder other than limescale on the heating element. Though I do run a cleaning product through about quarterly, I've never noticed reduced effectiveness between treatments.