a block of mysis is waaaayyyyy too much to feed the 4 fish you have. your defrosting method is good but you should only add what your fish can eat within a couple of minutes. use a baster or a pipette to suck up some of the food and squirt some of it into your tank. if your fish have finished all the food in less the a minute, add a little more and make sure they eat everything within a minute. whatever is left over should be taken out, although since you have a sleeper goby you could leave the food in there for a little longer.
but 1 block is definitely too much to feed the fish you have.