You will probably have to get a bigger pump. The rating is usually based on the depth of the tank since the lower in the tank you put the airstone, the less bubbles you get. I have a couple small air pumps for my 2.5 and 5 gallon tanks. If I put them on my 29 gallon and put the airstone on the bottom, I don't get any bubbles at all. Most of the good ones have a chart on the box telling you how many airstones you can run with it and how deep they can go.
I would also suggest that you pay a little more and get a good brand as they are quieter. All I own are Renas and they are extremely quiet. the first 2 I bought were so noisy, I took them back. The second time one of the employees suggested the Rena brand (he owned a couple). I actually took it out of the box at the store and tried it. As I say, I now own 4 of them.
