Intake fan for grow tent

I have a 3 X 3 X 6 grow tent set-up with about 300 Watts of CLF's. right now im still just a couple of weeks into veg but plan on adding another 250 watts during flowering. Right now i just have one small fan blowing cool air into a vent at the bottom of the tent and that might be enough, but once i add more lights i think i will need more.

My question is that my tent has circular 4" openings at the bottom and also at the top. I assume these are the spots for intake/exhaust fans. However all the fans I have seen come in square cases. Does anyone know of a circular 4" fan/case or do i need to try to modify the vent?

Also once they hit flowering i plan on using a carbon filter at the top to exhaust and hopefully contain the smell, but for now i just want to get more cool air in and moving around better.

You use ducting to connect it to usually an in-line fan, but if you dont wanna waste money on one, get a 240v fan that fits the ducting so you can tape around it or something similar.

Also I believe getting exhaust fans first is more important.

yeah...just by exhausting the air you're doing alot to pull more in...you doin a small HID?

Try get atleast, 1 exhaust fully hooked up and air-tight. Then you can add maybe one in-take, just to relieve some of the suction pressure on the exhaust fan.

There should also be some other holes in the tent to allow cables in and out.

The optimum setup is:

Air in one side, out the other.

---------------
|........... [OUT]
|
|
[IN]
---------------

Hot air rises, and cold air is denser and sits lower. The fan on the bottom should pull colder air into the tent and the fan up top should exhaust the warm air.

Good luck!