Frozen tuna steaks? Mahi mahi?

I recently bought some frozen tuna steaks, thawed them, then brushed with melted er, sprinkled with salt n pepper, and some other seasonings. I cooked them in the oven and it took them 20 minutes to get done. I loved the taste but after cooking so long they were tough? Is it ok to cook frozen fish to only medium or medium rare? I don't have a grill or a broiler pan...any suggestions/recipes?