Sorry to hear your lost, especially one at that size! Tigs do well for the most part but when they do have problems its hard to pinpoint...I've had a few tigs in my days and currently have one 18" and three 6" along with a 8" dourado catfish. From my experience tigs have a tendency to have non visible problems, internal vs external. Honestly I don't know what would cause your tig to swim in circles but usually any fish that swims in circles is more likely doomed! If your tig wasn't eating during the period it doesn't hurt to treat it with PRAZIPRO since its harmless and can be used as a preventative. My catfish wasn't eating for a whole month and when he did he would spit it out a few hrs later. Couldn't figure it out but then tried PRAZIPRO and it instantly worked and catfish was back to normal...like magic!! For future reference it doesn't hurt to try PRAZIPRO...it was a magic drug for me!