Aquarium systems powergravel vacuum cleaner - clean your tank of waste and debris without removing any water. With a powerful electric motor this has really good suction abilities, designed so you can clean the smaller tanks as well as the big, deep tanks due to the ability to add or remove one of the 3 pipes used for cleaning. Hang on the tank design with a mesh net the water must pass through before returning to the tank means almost all of fish waste and uneaten food stays in the net. Easy to put together, take apart and clean.
I brought this about a year ago when I had several tanks, this helped to keep the tanks cleaner in between water changes. Cost me about £50 however absolute bargain at £25 ono as I need to get rid of it.