I wouldnt necessarily use a PC fan for a 40 gal, I use a 4'' steel fan I bought at walmart for 4 bucks.... i clipped it to the side of the tank right under my lights and it works great...
but maybe multiple small pc fans would be a good idea?
However, the PC fans are electrically isolated due to the fact they are not line voltage... so there is a definite safety advantage to their use,
so if you do use the walmart fan, IMHO I would use ground fault protection (which your lights should be using anyways) and run a ground bonding wire between the steel fan and the plant rack
Always make sure everything is grounded properly, electricity will take the path of least resistance... be it a ground wire or you
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