Sump Question: Exhaust below or above water level

When I originally designed my plumbing, I had it plumbed so it exhausted above the water line, and into a sock filter. It turned out not to be very practical, as it was off gassing too much C02 for my liking and driving the PH too high.

I remedied this yesterday, by installing a longer piece of 2 inch PVC to extend below the water line, though still discharging into the filter sock.

The PH dropped, and that was great.

I expected the water level in the main tank though to rise, and was surprised to find that the water went lower. Significantly lower, like a couple of inches in the 280 gallon tank.

I am not familiar with the science of why this happened, can anyone explain?

Here is how we do it, and I think that you may be very well on to something with that, one drain is fully submerged, the other is half submerged. I believe that the submerged drain, is now under full siphon which has likely increased the flow. It's the only answer I can come up with also.

I am compensating by decreasing the flow so as to keep the water up to a respectable level.

If..I were to drill a small hole above the water line in the water exhaust, that would be enough to "break" the siphon effect? Not saying I want to, just wondering if it would. If I did, would a 1/4 inch be enough?

Thanks for your help, I drilled eight holes just above the water line of the sump and it fixed my problem. The water leaks out of some of the holes, but it has broken the siphon for sure.