I would agree with biffchicken. If your only growing a few plants you dont need one. If your growing a few marijuana plants, then yeah.. id say you NEED one. Or some sort of odour control.
I wouldnt grow even just 1 plant without a filter etc. If you are eager to get started but dont have the cash atm, you can still start to grow them in veg period ant not have to worry too much about smell, its when flowering kicks in. This will give you time to save up your buttons for a proper filter kit. But then if your closet growing you prob wont be havin a very long veg time.
So maybe that, or look into diy solutions which you can find around the net like darkdestruction420 suggested. Tried and tested. Some pretty ingenius ideas out there for not alot of money.
Maybe if you were oly growing 1-2 plants you could get away with using some other less effective methods than a filter. But 5 plants are gonna honk man.
Ona blocks work not too bad to a certain extent. That kinda thing.
These are supposed to be good
http://www.hg-hydroponics.co.uk/ona-breeze-fan--1-litre-tub-of-ona-gel-812-p.asp
But expensive. You could perhaps just buy the gel and mount a pc fan on top. Not sure how effective that would be though. And still, 5 plants is really pushing it mate.
