Bloodworms are a protein rich food, and will tend to constipate angels. I feed angels frozen bloodworms once every week or two, and follow with a feeding of frozen brine shrimp the next day. The brine shrimp has a lot of roughage, and helps keep them from becoming constipated.
Spirulina flake has a bit more veggie to it, as would some broken up algae tabs that are usually fed to plecs. I get a good veggie pellet from Angels Plus that I feed to all my fish, angels love the stuff. It has all sorts of things such as squash & carrots in it, I believe in as good a diet as possible for my fish, but I really can't turn my fishroom into a vegetable cellar as well.
6 liters is just shy of 2 gallons, I would do daily 50% water changes, if you are solving the problem with diet there is no need to medicate. I would keep the fish in there for a week, so you can keep closer tabs on the fish & diet.