could pc fans be used on a cool tube>?

if so hom many and is it worth it?

I think it wouldn't be efficient enough to get the job done which is cooling the light. If you add a carbon scrubber, forget about it. In order to cool the fixture you need to move lots of air into the fixture and then back out again quickly and efficiently.
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I tried cooling a 400W HPS 125mm cooltube with a 125mm 240VAC PC fan with no luck, tried two - back to back so both pushing air through tube mounted together, tried pulling air through with both mounted together, tried one either end of the tube push/pull which worked a little better but no combination could cool it properly. I got a 4" vortex fan with great results, it's quieter also.

3' x 3' x 8' veg cab temp went from 36 degrees down to 24 and the exhaust from the tube went from 48 degrees to 29.

Don’t even bother if you have a carbon filter or ducting over a meter they just can’t move the air.