Oh no! You probably just had an unlucky streak. I've chained fished twice, and got a shiny Clauncher at chain 37 that I traded for a shiny Skrelp, and I got your shiny Relicanth at chain 78, I think. I also found a shiny Foongus yesterday in a horde. It was random; I was just walking towards the Unknown Dungeon to catch Mewtwo when it appeared! I'm not really a fan of Foongus/Amoongus, so I might trade it for a different shiny.