My main reason for not eating fish is that I believe no animal has to die for me to live. There are plenty of alternatives to the nutrients found in fish without the cholesterol or risk of dangerous mercury levels. Protein can be achieved through beans, nuts and seeds and whole grains. Omega-3s can be obtained through adding ground flax seed to baked goods, oatmeal, cereal or smoothies. It is a cheap source (at least in my area.) A box of ground flax usually costs less than $2. A few brands of soymilk have added omega-3s and you can take veg sources of omega-3s in supplement form.
A second reason is that there have been new studies done that show that fish are capable of feeling pain. Check them out here.
The third reason is the one that I mentioned was very close to my heart. Even if you don't think fish are sentient enough to warrant concern surely you would think twice about eating animals such as dolphins (who have proven to be very intelligent) and endangered species of shark and turtles. Well, everytime you purchase fish you are supporting the killing of those animals and the destruction of the oceans. Commercial fishing practices often accidentally catch other marine species. Due to lack of space on the boats they are thrown off either already dead or injured. The practices used to catch fish, shrimp and crustaceans destroy the ocean floor.
Please consider at least reducing your intake of fish. You can improve your health, help the animals and help the earth at the same time.
Compassion Over Killing: Aquatic Animals
Shark Conservation: Dangers of Long-lining
Rethinking the Way we Treat Animals
Vegan Seafood recipes
P.S. If you love Sushi and have a lot of sushi restaurants in your area try the vegetable roll at each place. I find they are different at each place. It's exciting and delicious!