It’s not always true that water flows downhill.
What is true is “fluid will flow from higher pressure to lower pressure.” In order to achieve this equilibrium, water will do some pretty whacky things. What we want to do is connect the fish tank and the sump such that:
- Water in the fish tank will never siphon out completely
- Water from the bottom of the fish tank will leave the tank, carrying away solids
- No openings are large enough to ‘suck’ small fish out of the fish tank
- My ‘sump’ can rest on the same surface as my fish tank
Tall order! But it can be done. I want to acknowledge up front that I didn’t invent this design from scratch. It is a modification of the “Constant Height of Pond” or CHOP systems originally popularized by Murray Hallam, as so beautifully explained in this post by ecofilms: CHOP and CHOP2. My ‘addition’ is translating this into standard pipe sizes that can be found in the US. The 365 Aquaponics design is partway between CHOP and CHOP2 - like a CHOP1.5.
The drain out of the fish tank must be higher than the drain into the sump. If the sump is higher than the fish tank, you’ll never persuade the water to leave the fish tank. But the piping between the fish tank and the sump can bend or run under the ground if needed so the gardener can walk freely between the tanks. The three sketches below are “equivalent,” as far as successfully draining water from the fish tank to the sump.
But it isn’t enough to just connect the drain from the fish tank to the drain into the sump. I want to pull water from the bottom of the fish tank, where all the nutrient-rich sediment collects. So I’ll put some simple pipes into my fish tank to pull water from the bottom, as seen in this picture of my prototype:
One minor issue with using the standard, inexpensive, pipe sizes is that sometimes the water level in the fish tank rises faster than the water can escape through just the lower parts of the two pipes. In that case, I allow water to escape through the top of the pipes into the drain. Below is a video clip of me drilling the holes in the fish tank and sump and installing the bulkheads.