I'll set it up so a bell siphon will work in it if I ever add another grow bed, and my pump needs to be on all the time. But right now my pump only needs to run for around 10 minutes per hour to turn over the water, and fill the grow bed.
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The timer is simply any digital timer with a power point. You plug in your pump, plug the timer into household power outlet, and set when, and how often you want the timer to trigger the pump.
The aims are twofold.
1. To turn over 100% of your fish tank's water every hour (for the sake of your fish)
2. to fill the growbed to its maximum height at least once per hour, and have it drain back out again before re-filling (for the sake of your plants, and the nitrifying bacteria)
In my case, I estimate I'll be running the pump for around ten or fifteen minutes per hour, but that might require some adjustment after taking some water tests. It's difficult to estimate how much water my pump will actually shift in ten minutes, because I'm not exactly sure how high it will end up having to pump. The higher it's pumping, the less water per minute it will pump.
If this turns out to not work as well as I thought it might, I can always revert to a bell siphon, by simply replacing the standpipe with one that doesn't have a hole in it, and adding a bell.