hi the water enters via a wier a hole drilled in the main tank or via an overflow box then it enters the sump mine is split in to 3 sections the first is where the inlet pipe is the heater and skimmer the second has a deep sand bed in and some spaghetti algae in and the third has the return pump mine is classed at 6000lph also calcium reactor would go in the thired section if you have one
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