Many (like 30+) years ago when I kept tropical fish, I had a 'purpose built' vacuum cleaner. All you had to do was hook it up to the air supply from the filter pump to work it. The nozzle was shaped like a vacuum cleaner head and narrowed into a narrow tube. The air feed was fed in by the tube from the pump being located just inside the nozzle. All the crud on the bottom of the tank was 'lifted' with the rising air and deposited into a 'muslin' bag attached with a rubber band to the end of the tube, which was about 8" long. The muslin let the air and water out, but held in the crud. I've just looked on google to see if I could find a picture (but can't) and noticed there are loads about for £3-£10. Is it worth the hassle of building it yourself?