Flax oil contains the essential omega 3 fatty acid.
Fish/krill oil contains the essential omega 3 fatty acids EPA and DHA. EPA and DHA are the active forms in animals.
The problem is that humans have a very poor conversion rate of ALA to EPA and DHA. You want to be supplementing with fish/krill oil instead. Krill oil is basically made from tiny fish. It has a lot of additional benefits to regular fish oil. I use the krill supplement from n2bm.com every day. I highly recommend it.