Always make sure it is a good quality fish feeder too.

It depends on the type of fish – freshwater or tropical? Also the number of fish and the size of the food. Maybe get some advice from a pet store too?

Depends on how big they are. My daughter successfully used them for her fish but she was generally gone no longer then a week. But there are different sizes for different time frames

3 weeks is about the time frame for 2 fish. You could just put two in