Another way to reduce fan noise is to use rubber feet either side of the fixing plate and screw through them, using a suitable washer on the screw head side.

Also car acoustic deadening rubber will help with general vibration. Not sure if I'd put it in a tank, but underneath on the floor it should help. Example: eBay Store - Sounds Great: Dynamat Cascade: 10115 DYNAMAT Original 2-Sheet Speaker Kit Brand New

Also beware of corners in a room, these act like a trumpet and increase noise. If you can mount fans / ballasts away from wall and ceiling corners, or 1/3 way between small walls this should help.

John - ballasts will make more noise over time, if your's is a few years old, it may be worth replacing. Try mounting it on rubber feet, or a foam/ rubber base and aim your circulating fan towards it. A high density foam block, like a yoga block should help.