Supa 35 inch by 11 inch Under-Gravel Aquarium Filter is suitable for Tropical, Marine or Cold Water aquariums. Offer proof against “digging-up” by fish. Easy to install, has a high filtration rate and eliminates the filter wool layer. The air lift tube can be easily cleaned and positioned. Made by Supa in the UK in their factory in Sheffield. Directions: Place the under-gravel filter on the bottom of the tank and cover with gravel to a depth of 25mm-50mm (1-2). The weight of the gravel seals the edge of the filter plate to the bottom of the aquarium and water can only, therefore, enter the sub-gravel area through the slots in the plate. When in operation air rising up the airlift tube pulls water upwards from beneath the filter plate and this can only be replaced by drawing water through the gravel and filter slots. Over a period of time the filter bed matures and probably represents the best man-made equivalent of aquatic lifes natural surroundings. The filter is suitable for all types of aquariums including marine tanks. If higher turnover of water is required a second airlift or a power head unit can be fitted.