Get him a new hobby, I love fish stuff too but there are plenty of other things that would surprise me more at Christmas. Like a puppy
or more seriously a
Traxxas Summit RC 4WD Electric Monster Truck. I can't imagine anyone not having a tonne of fun that thing.
At the end of the day the guy sounds like he's got the aquarium stuff under control, he can obviously get whatever he wants without any issues. The best kind of gift is something he wants but has been told he can't have. If there's something you've put your foot down on in the past the best gift you can possibly give is a lighter foot.
Trust me guys are just like girls, they want what they can't have.