check with the pump maker or instructions, there is an adjustment on it somewheres that regulates the water. I think there are all a bit different.

If there is not an adjustment on the pump itself try one of these options: If the tube where the water comes out is within his mouth, purchase a "T" to diffuse the exit of the water. Our waterfall had water jetting out of the top and splashing out on the ground. Placing a "t" on the top worked for us and is not visible.
-OR- (best option) purchase a small valve to put in somewhere between the pump and the output. Then you just crank on the valve down to the desired gurgling flow.

Good Luck!

Be careful not to throttle the pump back too much or you will definately burn out the motor.

Depending on the type of pump you may be able to remove the dry end (motor and windings) and replace it with a smaller volume motor.

Steve