I use these for my Oscar all the time. I put a small amount of Aquarium Water in a small glass, and then drop in the frozen cube, it usually thaws enough, due to the warm water, in a short time and then I plop it in the water. My Oscar, Chumly, is a hog, lol, but he generally only eats off the top of the water or if something is floating down and he doesn't have to work hard for it. The beef usually stays in a cube shape, the shrimp separate more, I sometimes use a toothpick to loosen it up or split it into 2 servings.
Chum always puts as much in his mouth as he can possibly get, and inevitably a lot of it comes back out, broken up, as he tries to swallow it all. This works well for my tank because it's those pieces that are then the right size for my parrot and convict as well as my Placo and Raphael, which all scoot around underneath Chum at feeding time. I hope this helps, I love feeding them this way.