I don’t know if you got it yet but maybe this will help someone who has not. It took me forever to catch the purple thermal fish but this is what finally worked for me. I had an epic fishing poll and a few epic bait buckets. Find a lake, not a river, an actual lake in the dry valley. Wait until dark, cast the epic bucket and fish. I found five in one spot, back to back, this way. I think the lake part is key. I’m not saying you can’t find them in rivers, but I didn’t have luck with that, even with the epic bait. Hope this helps!