i think your sand filter is a great idea but one problem the diameter of the pipe is to small
the problem with all sand filters is when you turn the pump off the sand compacts down very tight and its near impossible to get them restarted without turning it on its side and the diameter will make this even harder with the amount of sand you have in it
i made my own to start with but it was a total wash out
so i brought this on which is a great design if you want to copy it as it has a bypass so you can put a very big pump on it and only use the power you need for the sand filter the rest will flow back to the tank as normal it was gives you the advantage of having much much morepower from the pump to get it moving again if it compacts dow
you also need to make sure the water going into it is super clean no particals or it will turn into one big crap filter which will just sit ontop of the sand which you wont be able to see as the pipe is not clear you also need clear pipe to see the level of the sant moving
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this is mine i run a 18,000 lph pump throw it then the bypass feeds the UVs