Good brands independently list how much EPA and DHA are in each pill. Bad ones say "1000mg". That is useless, you can end up with minimal EPA and DHA and a bunch of filler oils. Meh.
Best I've found is ~600mg EPA and 400mg of DHA per capsule.
Edit: Good brands are purified, heavy metals should not be a concern.
Well be aware, 300mg of EPA means the value isn't quite as good as it may first appear. Multiply by 2.3 to get the dosage the same per $!
I have never used Swanson stuff before, typical worry about low cost fish oil is that it isn't filtered for heavy metals as well, and it may have a smell to it. Of course YMMV. :) I typically go to a local shop (more expensive of course!) and try out a brand there first, since I typically can return even the opened bottle if I feel something is wrong.