Ok so someone told me to get a 16'' and a 6'' for my grow tent is this correct? also does it mater where they are like witch corner? Also on Inline Fan what do i need? im getting a 5x5 tent for 4 plants under 600w HPS but HTG Supply - 8" Inline Duct Fan is a lot cheaper then all the other 1s like HTG Supply - GrowBright 8" High Velocity Inline Fan is the 2nd Cheapest at 8'' What is the huge difference?
Don't know what your first question is asking, but for your second question, just read the info on the fans you linked. Straight from the pages you linked: 8" inline duct These economically priced fans have a lesser cfm (cubic feet per minute air exchange) than the high velocity fans also available on this site. Perfect for air cooling a single lighting reflector or ventilating a small closet sized grow room. *Can not be used with Blower/fan speed controller, not compatible with carbon filters* 8" High Velocity Inline Fan This is not a wimpy inline duct fan. this is a high powered, high velocity Inline Fan that is very powerful and moves a lot of air! This is an excellent choice for ventilating grow rooms / grow tent / grow cabinets and is perfect for use with carbon filters. So basically, the first fan is only good for venting heat/exchanging air, while the other is able to pull air through a filter.
You want the high velocity if using a carbon filter. If you get the inline duct and put a filter on it, you will probably have heat issues. If you get the inline duct and don't use a carbon filter, your plants will stink up your whole place. Best bet is to get the high velocity instead of wasting money on the inline duct.