There are basically three reasons to use fans: 1) cooling (which you don't seem to need); 2) recirculating fresh air so that the plants get as much co2 as they can absorb; and 3) odor control. You don't seem to need fans for cooling, and if odor control isn't an issue then I'd just try it without and see if the air, by itself, gets recirculated enough for your plants to breathe and not suffocate. If they die or look like crap, then you'll need to do something to give them fresh air. But they might be just fine as is.