I agree with the answer above.
They are gentle, timid, and very nervous kind of fish highly prone to being spooked, darting and bashing into walls and thus injuring themselves, sometimes killing.
They are one of the most active swimmers among catfish and hence require 1.5 times more space than other same size cats at a minimum.
Small tank vs big pond makes a LOT of difference in regard to their overall comfort and the tank mate selection. I have had about a dozen of IDSs in my 4000 gal pond with a 100 of other fish with no major issues. See 5 pages of photos here, if you will, to see what pond mates I have had with mine:
http://www.planetcatfish.com/forum/viewtopic.php?f=25&t=29488&hilit=+pet+catfish
10 year old IDS that's 15" sounds strange. All of mine and of other keepers I know hit 2' in 1-2 years easily. The poor thing must have been terribly stunted.