Definitions of some terms!

[JCharles]

So for the toothpick thing, imagine you've turned all your flow off and are dangling a toothpick on a piece of wire in your tank. It will be straight up and down, dead vertical. Call that a 0 or 1, no flow.

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Now if you blast a powerhead right at the toothpick, it is going to be dead horizontal. Call that a 10, or max flow.

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If you're holding the fishing wire, you can move the toothpick around the tank with your flow on and observe how diagonal it is. You only have 90 degrees of movement between fully horizontal and fully vertical, so 45 degrees would be a 5 on the scale or 'moderate' flow.
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..\ This would be a 3, ignore the dots.
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Ive also seen flow described as a function of total tank volume. Low flow being 40-50x your volume and very high flow being 100x (please check those numbers before doing anything, I'm not 100%). So you would want to add up the gallons per hour of your return pump plus any powerheads you have, then divide it by your total volume.