400w cool tube

How strong of a fan would I need to keep a 400w hps cool.

depends on if you are using a carbon filter, using a grow tent, where you live, relative ambient temperature, etc

No carbon filter a 4x4x7 grow tent

i use a 6inch with carbon filter and iv neva had a problem

What cfm output?

Depends on the ambient temperature of the room, but if you are keeping it around 70F anything greater than about 250CFM will keep you less than 80F which is usually ideal

ruck 150mm/6" 470m3/hr

Sweet just got a tent and a 400w cooltube trying to get the setup ready. How many plants u guys think I should keep under the 400w hps?