I use these cubes all the time. After you let it defrost for a little while, I place it on a cutting board and squash it with the side of a knife. Then I chop at it a little bit. Yes, it is a little more work than most foods, but I still give it to my tangs at least twice a day.
Tangs are kind of specific in their diets. If certain tangs lack certain algea in their diet, they will slowly become malnourished and decline. (the naso tang for example, does best when it has brown algea in its diet)These cubes cover a large portion of marine plants. Adding to that nori, spirulina flakes and home grown macro algea will help out a lot with keeping tangs healthy. Tangs do not eat much meaty food in the wild. While you can and people do keep tangs for long periods of time feeding only meaty foods, they tend to do better, live out more of their life span and act more naturally when fed various types of algea.