I was puzzled, and I feared my fish were sick or just terminally picky. After all, I had done my research before buying my fish. Porcupine puffers (Diodon sp.) should eat mussels, shrimp, squid and clams, right? Sure they do, but mine wouldn't touch anything already prepared from the store.

I had an epiphany when I was thawing some mussel cubes. They looked nothing like the mussels I make with pasta. So, I went to the grocery store and bought five or six live mussels from the seafood counter and took them home to experiment. I was naturally quite cautious. I put the live mussels into clean saltwater made up with marine salt mix and let them sit overnight. All were still alive the next day, so I plunged ahead.

My porcupine puffer needed to eat something — anything. I cut open a mussel, cut up the insides with a sharp knife and nervously fed just a pinch to my picky puffer. It gobbled it right up and swam back and forth looking for more.

I went back to the seafood counter and got a single raw, shell-on tiger shrimp, a live littleneck clam and a quarter pound of raw squid. The puffers greeted the new food with great enthusiasm when fed a pinch at a time.

Don't know what to buy? Look up your fish or invertebrate species in a reliable reference book to determine their natural diets. Then buy the fresh or frozen foods in those groups.

• Crustaceans. Shrimp, crab, crayfish, lobster and prawns. Keep the shells on.
• Mollusks. Snails, clams, oysters, mussels, cockles, scallops, (all of which, when bought, should be tightly closed — a sign that they are still alive) as well as
squid and octopus.
• Fish. Filets of nonoily marine fish such as cod, haddock, pollack, halibut, red snapper and so on.
• Vegetation. Dulse flakes, nori (sushi seaweed), kelp, laver, sea lettuce, alaria, Spirulina powder. These can be purchased online, at a local health food store or at an Asian market.
• Miscellaneous. Sea urchin roe.

But what about the vitamins and minerals in prepared foods? Get a good grade of marine fish vitamin and HUFA (highly unsaturated fatty acid) supplement, such as a combination of Selcon or Zoecon and Zoe supplements, and add them at the manufacturer's recommended rate to the food you make.

You may think that the little cubes are so convenient, so you won't bother making your own food. This is certainly understandable, but if you're so-inclined, you can make your own cubes with a few mini ice cube trays. Shuck and scoop the flesh out of any clams, mussels, snails, cockles or oysters you will be using. Chop up a selection of mixed food items representative of each species' diet using a food processor, blender or simply a good, sharp knife. I prefer a food processor for speed and ease of cleaning.

Press plastic wrap down onto the top of the "ice cubes," then wrap the tray in plastic wrap. Freeze until rock hard, take the cubes out of the tray and seal small portions into freezer bags. Store these in the coldest part of your freezer. When you want to feed your fish, simply pull out a cube and defrost just like you would a commercially made cube.

Even though it isn't necessary, you can create a cube that retains its shape after thawing. Add agar agar (an algae-based gelling agent) to the mix. Agar agar can be found at Asian grocery stores, health food stores and on the Internet. Most brands will have mixing instructions on the package.

Some fish, such as puffers, need to wear their teeth down. Much like rodents, some fish's teeth continue to grow throughout their lives. Whole or shell-on foods, such as mussels, clams, snails, crabs and shrimp, will do this nicely. Snails, mussels, cockles and clams can be cracked open just a bit by wrapping them in a towel and hitting lightly with a hammer, rolling pin or a can of food from your cupboard. Shrimp and crabs can be cut into crunchy single-serving portions with a sharp serrated knife or kitchen shears. These crunchy foods can also be frozen into vitamin water in ice cube trays to store nicely in the freezer.