[Zuider]

You can use the Polynesian palm scallop, which attaches itself to the damp bathroom wall in picturesque clusters like roof shingles. It is an aerial filter feeder, capable of subsisting on household dust (which is mainly organic matter) though for faster growth, one can use a fine-mist spray of blended sausage meat and soggy marrowfat peas. They are also excellent for repelling rats, their natural predator, by emitting sharp bursts of ultrasound when the rodent's presence is detected.

The problem is in getting hold of them, as trade in this protected species is illegal.