When I had my biocube, I replaced the stock ones with SilenX fans. They move more air, and are loads quieter.
It's a super easy mod. Just take off the light cover, remove the 2 screws holding the fan inplace, unplug from circuit board. If your new fan isn't a 2 pin plug, just cut off the old plug and wire it up(black to black, red to red). The third wire isn't needed as its just a speed controller based on temp typically used on a PC motherboard.
Any 50mm PC fan will work. Just look for one that moves more air(CFM) with less noise(DBa).