Sorry to hear about your cowfish Randy. I fed mine like a puffer..occasional food in a shell, such as cherrystone clams w/ the shell cracked a bit. I know they like a good bit of algae in their diet too. I had to euthanize mine as well, he just started spinning one day.
And he was doing so well. I took him is as a rescue from some nice folks that kept him with a blenny that ripped up his tummy.I spoke with a lady at the Tampa Aquarium about his nutritional needs, she told me about giving him shelled food items, but warned about internal parasites being the norm with cowish.