I've never been able to find something off the shelf that works very well. I take a piece of aquaclear sponge or nylon scrubber, tie it to one end of a length of monofilament line. I then tie a couple nuts or some other kind of weight to the other end. Pull through your tubing a few times and it's like new. I use these to clean the u-bend portion of filter intakes as well, the flexible brushes I can find never seem to be flexible enough. If you've got a pressure washer, it should clear it out pretty quick, too.