those blocks have plaster or ash or something in them that is not good for fish. most fish can go several days without food, or you could get someone to feed them for you but if you do you should probably leave premeasured cups of food so they don't overfeed them. hth
Most fish can go up to two weeks without eating. If you're worried about leaving your fish without food that long, get a neighbour to feed them once during the week. Leave the food premeasured, as karrihug said, so the fish aren't overfed.