Yep, about 2" or so down, slightly below the venturi. this will prevent the tube from inverting as well as gas locking the powerhead.
It'll also leak out excess CO2 if you add too much.
By directing the outflow from the powerhead to hit thwe side of the reactor tubne instaed of shiooting out the bottom, you no longer need a sponge.
The tiny bubbles that are expelled get dissolved and greatly increase the ability of the reactor to add more CO2 to the tank. It might seem wasteful intially, it is not.
This only occurs after the CO2 levels rise high enoyugh to build up a gas bubble and then when it gets to the predetermined venturi level, it starts pumping the gas out even more and wasting it some.
The O2 build up is removed this way.
Yes, the plants are doign much better from the sounds of it.
Remember the plant's response now.
That's what you want to see, if not, check the CO2.
The nutrient routine is easy now once you have good cO2.
Keep it up.