| Filtration Biological filtration in any fish tank is essential. But in cold water the bacteria don't work as quickly as they do at tropical temperatures. This can lead to problems because dirt that should be broken down by the bacteria is left in the tank much longer and this gives other more harmful bacteria a chance to multiply. In this type of situation all will initially seem well but the fish will be under stress, and the bad conditions plus the stress will lead to ill health, usually in the form of finrot in Fancy Goldfish. U/G filters (under gravel), these are extremely efficient and unobtrusive, but because the water is drawn through the gravel there is a tendency for dirt to become lodged between the gravel. This should not be allowed to happen because it will reduce the flow of water and the dirt will be left in the tank and problems will start. If you use this type of filter with goldfish (Which are messy creatures), the gravel must be raked through once a week and the dirt syphoned away. If this is done there will be no problems. External Power Filter, probably is the best choice, as long as you select the right size for the tank. Again because of the amount of dirt these fish produce it will need regular weekly maintenance. 
By arranging the filter media as shown the filter will remain free from clogging for much longer. The sintered glass media is one of the best available for bacteria to colonise, it is quite expensive but only needs to be bought once. More information on filtration
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