Powerhead Blowing Too Much Air

HI fish lovers I'm not really good in English I hope I can explain my problem well. I'm using a top filter aka overhead filter on my 20 gallon aquarium it is powered by a powerhead. The pump is blowing too much air for my pristella tetras. My Pristellas are getting blown to the glass of the aquarium and one of them broke its mouth. My question is can I cover the hole where the air comes out? Thanks lots

 

pictures?

 

HI I can't take pictures now because I'm not at home. Do you think I can close the hole where the air comes out? Thanks.

 

probably not. I'm assuming that would cause a back up on the power head and cause a failure

 

What can I do to weaken the air coming out without breaking my pump? thanks

 

you can put some wood or heavy decorations near the exit to maybe diffuse the flow