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Undergravel Filter

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An undergravel filter is not the choice of most goldfish keepers. The goldfish produce tremendous amounts of waste. An UGF processes this waste using the biological nitrogen cycle completely - no mechanical filtration really, at all. Most UGF are not up to the job of goldfish waste. Some here do like them and use them, usually with a secondary filter that does the mechanical filtration of the large waste debris. There is a thread that discusses this rather nicely.....


That said, sand is also not a very good substate in a goldfish tank. It would fall/suck through the slots in the UGF and completely fill in the space under the plate where the waste is being broken down by the beneficial bacteria. Take away this space - where oxygenated water circulates, etc - and the nitrogen cycle ceases and the UGF will not work.

Sand is also not a good substrate for goldfish because they are bottom feeders - they like to root around, digging on the bottom of the tank looking for food. They would constantly stir up the sand - causing a sandy haze in the water at all times. As they dig, they swallow sand, passing it out through their gills as they do. This action can be likened to sandpaper on the gills - very irritating and not good for the fish.

Very small gravel - small enough not to get stuck in a goldie's mouth, or very large gravel or stones is a preferable substrate for such a tank. :)

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