I personally have not used them but I'v heard only bad things about them and my grandpa used them but then swithced to
HOB (hang on back) filters so further proof for me to believe they are bad. They suck the junk under the gravel and it then eventually gets stuck there and creates
ammonia and other problems. Yes they do run 24/7 and yes it's fine to let them fun 24/7, if you don't it's not good as the fish need to be in constantly filtered water to keep the tank cycled (check the
nitrogen cycle in freshwater begginners if you don't know about cycling) In a undergravel filter it's not sucking down that much but the perferred and much better filters are HOB either aquaclear or bio-wheel and if you really wana spend some money a
canister filter. The HOB filters let you choose the type of filter media you want and when you want to change it and are easy to operate and clean. I would advise not getting a undergravel filter.
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