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When talking fresh water fish tanks, why do people recommend that fluidized sand filters have a prefilter like a sponge? Assume there is a separate system for removing solid waste from the tank, like a hang on back filter filled with sponges that are easy to remove and clean while the system is still running.

Having a grate over the pump intake would keep out any item too large to pass through the pump, and anything smaller is going to be fish/bacteria waste or some small part of a plant that broke off. So whatever is sucked through the grate on the pump intake needs to be broken down or physically (read manually) removed from the tank.

Given that we are assuming there is a separate system for physically removing solids from the tank, it is not the job of the fluidized sand filter to do that. So we are only concerned if the solid waste will hinder the operation of the fluidized sand filtration or adversely affect the tank in some way.

Anything light enough will just pass right through the sand and flow through the outlet, which should have no effect on the effectiveness of the fluidized sand filtration. Anything too heavy to flow out will remain in the sand until it is broken down and either liquefied or shredded in to small enough pieces to flow out of the filter. Any of those cases should not interfere with the filer.

Yes I know, having things breaking down in the tank/filter instead of manually removing them will put a heavier load on the bio-filtration capacity of the whole system. But it is only a numbers game, simply buy less fish and have a large enough fluidized sand filter and that should negate the increased load issue.

Edit: The reason I have set up a fluidized filter is to better utilise the powerhead I have. For not just water circulation in the tank, but for venturi, surface agitation aeration, and the fluidized filter for nitrification. I have made a DIY fluidized filter by using a drink bottle, some pipe, a tee piece to allow some flow to bypass the filter and a tap to control flow rate through the bypass. DIY fluidized sand filter

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  • Can I ask why you're considering a sand filter for a fish tank. I have never in almost 30 years seen these recommended for hobby or even small commercial tanks. The issue with closed freshwater systems is they essentially cannot facilitate denitrification no matter the size of filter or bio load. So no matter what you end up with an excess of nitrates which must be removed via water changes. A sand filter is complicated and unnecessary compared to other biological filters. Some have successfully setup denitrification but it's not easy in fresh water the same as saltwater.
    – Jestep
    Commented Dec 19, 2017 at 16:41
  • A traditional sand filter is only fluidized when back flushing for cleaning. They don't need special prefiltering. It sounds like the prefilter / strainer is just to protect the pump. Commented Dec 19, 2017 at 21:19
  • @blacksmith37 I'm not talking about sand filters that are not fluidized, as they are mainly concentrating on mechanical filtration and not performing as much bio-filtration as a fluidized one, and they require periodic back flushing to clean. Commented Dec 20, 2017 at 1:26
  • @Jestep I know the fluidized filter will give no added nitrate removal abilities, but the small hob filter is not enough nitrification for the fish load I am intending, and I had the powerhead anyway. The amount of water changes I need to do will, like you say, be the same, with or without the fluidized filter, due to the need to remove nitrates from the tank with water changes. Personally I don't think a DIY fluidized filter is that complicated or expensive compared to any other type of filter, so I made one. I just don't want to have to dive in to the tank to clean the sponge on the intake. Commented Dec 20, 2017 at 2:55

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