With the open ended tube, you're not building pressure so it will release just as mentioned. If you are using a drop checker, then you could get a feel for how much is getting dissolved. I have a similar situation with an old limewood airstone. It has developed a hole due to all the the cleanings. The bubbles come out of the hole just as you describe. There's no pressure to keep it back and force through the smaller pores.

It will be interesting to see how the rice performs going into week 3. I have this bottle on a 20high.