I discourage small tanks because, as you may know, they are very very fast and quite active, they would be totally unhappy in a smaller tank. A 90 (im buying one this weekend for marine
) is only 18", a shovelnose cat grows past that. I tend to suggest having a fish no more than half as wide as the aquarium, so it has lots of room to swim. Shovelnose cats are not inactive fish, if they aren't swimming, its because they are unhappy with the tank.
I work at my LFS, one of the largest fish stores in the world. We get adult shovelnose cats in, and our tanks are only 18" deep (and around 150 gallons each... we have 300 of them or so), and they don't even turn around in the tank all the time we have them.
Tiger shovelnose tho,you just even mentioned it, Ive kept them too, they feel like they could use a bigger tank than 480 gallons (8 feet long x 4 feet wide x 2 feet tall).