My lo has had fish from 6 months and loves it. It's great because it's easy to mix in with things like pasta and slips down easily! There are some nice recipes in the Annabel Karmel book. I have no idea about shellfish allergy and when to introduce fish. I would check with hv or gp just to be sure. We haven't done shellfish yet but I think you can from 6 months too.
Oooh found this on baby center website:
Crab, shrimp, and other shellfish make the American College of Allergy, Asthma, and Immunology's list of most common food allergens, so many experts recommend that you introduce these foods slowly and only after your child's first birthday, when his immune and digestive systems are more fully developed. "If your family has a history of food allergies, hold off until at least age 3," says Marilyn Tanner, a registered dietitian at St. Louis Children's Hospital in Missouri and a spokeswoman for the American Dietetic Association.
Signs of a food allergy include swelling of the tongue, lips, and face; a skin rash; wheezing; abdominal cramping; vomiting; and diarrhea. If your baby develops any of these symptoms after eating shellfish or any other food, call his doctor as soon as possible.
That said, keep in mind that only 4 to 6 percent of infants and young children develop a food allergy. And if you lived in Asia or a Scandinavian country, where fish is a dietary mainstay, your baby would probably sample a variety of fish even before eating meat or chicken. But in the United States, it's customary to introduce fish and shellfish after a baby has accepted both pureed fruits and vegetables and strained meats and poultry.
If you have no family history of shellfish allergies and you want to try feeding shellfish to your baby before 12 months, wait until he's at least 6 months old and start with shellfish that's easy to puree, such as crab or lobster. Wait until he's able to chew other finger foods successfully before offering him fingertip-size pieces of shrimp and other shellfish that are harder to puree. Introduce only one type of shellfish at a time.