The reason you are cycling so often is that your siphon is rather large for the size of your grow bed. The siphon you have designed has about a 2:1 timing range. As you have noticed as you adjust the water flow the timing changes until you bump up against the limits of that range. In order to increase your time you will need to reduce the size of the stand pipe. This will decrease the rate at which the water flows out of your grow bed. You will also need to decrease the inflow as well so as not to go beyond that new limit.

As in most things in aquaponics, there are differing opinions about how things should be done. I would say that cycling as fast as you are is probably too fast, as it does not allow for the moisture that is retained by the media and plant roots to have time for the oxygen in the air that is drawn in during the drain half of the cycle to fully penetrate that moisture as far as it otherwise could.

In using our PinPoint DO meter, calibration requires placing a film of moisture on the probe tip and allowing the oxygen in the air to be absorbed into it. It takes only a few minutes for it to fully saturate that thin film as the meter changes and then stops changing. That is an indication of how long it takes for the air to saturate the moisture left behind on the media and roots.

So, I would say a 5 to 15 minute cycle would be about the range you would want to design into your system. Anything less than 5 minutes and the aeration of the grow bed may not be complete. Anything longer than 15 minutes is decreasing the amount of nutrients that are presented to the plants and the media where the bacteria do their work and require lots of oxygen and waste to be processed.

Having said all of that, I am of the opinion that if you have enough dissolved oxygen in the fish tank water, then all the above doesn't really mater.

Again, this is just my opinion and I will admit that I am shooting from the hip here.

Oliver