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I rescued an 11" irridescent shark (Pangasianodon hypophthalmus) from a 55 gallon tank about a week ago. The fish seems to be in good health other than the obvious signs of being cramped in a way too small tank. I have it in my 240 gallon display tank and it seems way too calm. I have two smaller of the same species and they dart around and try to swim upstream against the output from the filter. This big guy just kind of hovers in place and also doesn't seem to feed. I've tried floating pellets, flake, and even tried hand feeding some peas but nothing is readily eaten. So far it looks like it has not lost any girth, the stomach is flat but not sunken in so I assume it is getting something to eat when I'm not around.

I hope this is just a case of new tank jitters but any advise would be great.

Some of mine 1'-2' ones would take anywhere from weeks to months to start feeding well while others would start in days. I have not noticed any correlation. I have had about a dozen of them. One of them, the "Big guy", the oldest, ~15 years old, would not eat floating pellets for 3 months or longer; he would not eat anything else either at all. I had to buy the sinking type just for his sake and then he took them. Still always ignored any other foods.

One week is not long. Give him time and keep a close eye.

Thanks for that info Viktor, I watched the fish for a couple hours after lights out and no feeding behavior so it's good to know it might just take a while. This morning I saw it trying to swim and it almost seems like it is affraid to get any speed. It pumped its tail pretty hard a couple times but then slowed and stopped. It would turn and do the same thing, I thought it odd until I noticed it always stopped at the same spot, the length of a 55 gallon tank (36"). I can't wait for this poor fish to notice it has more room to swim about.

Think about it, it was stuck in a glass box to small to move around. You'd be hessitant to to try to swim when all you knew before was to do so meant a headache.

At some point he'll get it the tank is a lot bigger than it used to be. Noticed the same behaviour with my 15 cm Ancistrus Cirrhosus when she got transferred from a 60 cm to a 125 cm tank. The critter thought she was in the corner already but she was only halfway.

what'd be cool if someone captured that "'d been in a small tank for too long" syndrome on video... not an easy thing, I realize, you'd probably have to be a little young Spielberg armed with patience and film montaging technique...