Fabric softener leaves residue in the flush-through dispensers on my Neptune TL and F&P AquaSmarts. The AquaSmarts flush and drain the softener dispenser five times, still leaves residue that needs to be occasionally cleaned. I don't use softener on every load but eventually there's enough accumulated residue that cleaning is required. I can tell when cleaning is needed because the siphon stops working and the dispenser remains full of water.