PC fans are not a good choice unless you are using under 100-150 watts of light, and even then there are more appropriate fans for the job. My PC has three fans. If my grow space had proportionately sized fans there would be three ten " axial fans. Instead I use one six " inline fan and have more ventilation at less watts used. What size cab, how much light, what is cab made of, what are ambient temps in room cab is located in?