UNDER GRAVEL FILTERS WORK WHEN WATER IS DRAWN THROUGH THE GRAVEL. AS THE TANK WATER MATURES BENEFICIAL BACTERIA REPRODUCE AND SEED THE GRAVEL. THE BACTERIA LIVES ON THE SURFACE OF THE GRAVEL AND FEEDS OFF THE NUTRIENTS IN THE WATER FILTERING IT OF HARMFULL NITRITES AND AMMONIA. UNDERGRAVEL FILTERS WERE PARTUICUALRLY USED IN MARINE AQUARIUMS. THE ONLY PROBLEM THAT THEY CAN SUFFER FROM IS THAT THEY BECOME CLOGGED AFTER TIME WITH MULM AND DETRITUS.HOWEVER ,WHEN USED IN REVERSE FLOW THIS PROBLEM IS ERADICATED. YOU CAN BUY VARIOUS TYPES OF UNDER GRAVEL FILTER OR YOU CAN MAKE YOUR OWN. I HAVE NMADE MY OWN USING PLASTIC TIOLET OVER FLOW TUBING ABOUT 1 INCH IN DIAMETER. THESE CAN BE GLUED TOGETHER IN A SHAPE TO FIT THE TANK YOU HAVE AND THEN WITH ONE UPLIFT TUBE IN THE CENTRE. HOLES ARE THEN DRILLED IN THE TUBING ON THE BOTTOM FOR THE WATER TO FLOW OUT OF. (SEE THE PICTURE BELOW). YOU MUST ALWAYS USE A GRAVEL TIDY TO PREVENT THE FISH FROM DIGGING DOWN TO THE FILTER PLATE.ONCE THE FILTER PLATE IS EXPOSED IT WILL NOT WORK. GRAVEL TIDIES CAN BE PURCHSED OR MADE FROM PLASTIC MATTING WHICH YOU CAN BUY AT MOST GARDEN CENTRES.

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