These look like bits of plaster but they seem to have food in them. I don't know how good it is or how the fish know when to look for it but at least the fish survive while you travel. The battery operated timed release ones are probably a better option since you can feed the same food as usual and they hear it so they know to eat.

Pros: feeds the fish while you can't

Cons: who knows how well?

I have never had much luck with these and don't recommend them. Anyone considering one of these should consider purchasing one of the battery or electric powered automatic fish feeders. They will provide the amount and kind (dry) of food that your fish are accustomed to. It is a much healthier choice and one which will ensure that your fish will still be alive when you get back from vacation.

Pros: Can't think of any.

Cons: pollutes the water.

Nice thing to have when you go on vacation. Plus they don't cloud up the tank

Pros: keeps fish feed

Cons: none