Honestly here is the best way and yes I've done this more than 10 times. Compltely empty the tank, pick up a bag of miracle grow organics pitting mix (see pic) and add about 1 1/2 - 2" in the bottom but make sure you break up the large chunks. Flatten it out and make sure its level. Now add your prewashed sand or fine gravel (dont use normal gravel, its to large and the soil will pass through it) to the top. About 1" - 2" and level it out. Kind of push down on it so its a bit compacted (you will tank me later for that tip) now place a plate in the tank to aid in not splashing everything around and add water extremely slow. Once filled it should look really clear.
I have never poked the soil either and I don't get air bubbles at all escaping.
Miracle grow organics
Pic taken as soon as I added the water
Been running this one about 6-8 months