There are plenty of ways to make DIY carbon filters but I think the easiest way hasn't been mentioned yet. Take a 500ml coke bottle, cut the ends off so you have a plastic tube. Then get a sock and stuff it with activated carbon, stick the sock in the tube and you've got a filter. You can use PC fans for intake and output as well, just scavenge/buy some pc fans, scavenge/buy a 12v charger (phone charger ect.) connect the posotive wires from the fan to the posotive wire in the charger by stripping the cover off the wire and connecting them together with tape or by twisting them together. Then do the same with the negatives and the fan should run perfectly when plugged in. You can connect several fans to the same charger in this way. Some fans have a third wire for speed control, you don't need this so just chop it off.

simple, i like it

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Come on does that actually work? I went all elaborate with mine, spent about $50 on crap for it, ran out of money, and still havent finished it. I tell myself im waiting on my fan to come in the mail (stupid brown not working for me right now because of the holidays, sorry UPS)

Yeah, it works, the fans aren't that powerful but there cheap and easy to find so you can use loads of them. I have a small grow space so I only use 4 but thats plenty, and i got everything for free except the activated carbon.