When and how to start using a fan inside indoor?

[xxorezxx]

As the title says, I want to strengthen my girls through the use of a Fan, but when I used it for the first time as they had like 7 days old I caused them a little windburn in few hours, so now I'm a little afraid of turning on the fan.
Now they are on day 16.

 

[mr buddy]

What size fan ..have one on but don't aim directly at the plants have it aimed at a wall of the tent so it bounces off onto them

 

[cannabisweedPotNerd]

From start

 

[cannabisweedPotNerd]

Like he said don't have it blowing at them

 

[xxorezxx]

Like he said don't have it blowing at them

Thanks a lot !