I recommend you put an overflow pipe in each of your tubs that sends the nutrient back to the reservoir. that way it will never overflow and the 15 minute increment should work fine.
Overflow pipes are pretty standard in EnF systems. you really shouldn't build one without it. (what happens if the timer fails in the closed position and the pump doesn't turn off)

I agree about the overflow safety pipe.

It sounds like what you want is a cycle timer. I put a link to some of them below.

Here is a link that might be useful: worms way timers

Thanks for that. All tubs now have overflow pipes attached and timer works perfectly.
Dan

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I agree with the Over Flow pipe and might add a metering Valve of some type to restrict GPM flow. Most pond pumps are simple centrifugal pumps that won't build pressure. That is why they have Height restrictions. A simple in line Ball valve should do the trick.

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