Frozen bloodworms are an excellent food for aquarium fish, especially large fish, or juveniles in a growth phase. However, I think it is too easy for many aquarium owners to make mistakes with them and end up with bad results.
Bloodworms are a rich food, and it is very easy to overfeed. The amount that comes in one of those little cubes is too much for most medium-sized aquariums. Accidentally overfeeding fish can lead to a host of fatal diseases, and with a rich food like this, the process seems accelerated.
The little-frozen cubes should be defrosted before feeding. I just put them in a small cup of water and let them sit until they are fully defrosted. Never simply toss the frozen cube in the tank. The fish will pick at it until it is gone, but frozen food isn’t the best thing for them.
Once the bloodworms are defrosted in the cup, it is important to rinse them and get rid of all the debris that came packed in the cube. This isn’t necessarily bad for the fish as it is just tiny pieces of bloodworm, but it is too small for them to eat. So, it ends up fouling the tank water.
Used correctly, bloodworms are a great source of calories for tropical fish. Make mistakes, and you could end up with sick fish. Unless you are an experienced fish keeper, I advise only offering bloodworms as an occasional treat.