So sorry for your loss man. I've kept mine in a 75 gallons until he reached 12" then moved him to the 3k. For all it's worth, I've always fed him well and gave ample swimming space (maybe not when he reached over 30").
Here's something i've done that i've yet to notice from others. Mine have always had a pretty strong source of current. While in the 75 I got him with a 2600 lph powerhead. Upon moving to the 3k, i have both a power head, a pretty powerful wavemaker and a really powerful return pump. in addition to that, i've got 3 more pumps on places in the tank that i thought would not have as much circulation as i would like. Hence, i would say that there was water movement (in varying intensities) around my tank. I've always noticed him swimming against the current in front of my wavemaker. from there, he would occasionally go down an "park" at the bottom of the tank then go back to swimming. He is one of my most active cats.
I feed him a lot (usually during the night) but the big bulge in his tummy is gone the following morning. for what it's worth, i'm thinking this is a catfish that likes to be active and maybe being too docile does not serve them well.