Use a firm white fleshed fish. Find a reputable fish monger and tell them you're making ceviche and listen to their recommendations. Ask for the freshest salt water white fish they have. And a fish that's been frozen. Freezing fish helps kill some of the parasites. Lingcod, Flounder, sea bass, and halibut are good choices. Avoid oily fish like tuna, salmon or trout. Avoid cod.