Depends on how much time they spend with the animal and whether or not they were staying at your house. Normally someone that stays at the house or the animal(s) went to their house for the whole amount of time would get paid more vs someone who only went over a few times a day to checkup and feed the things.
that depends on how much time is involved. Fish are easy, cats a little harder and some breeds of dogs need a lot of attention. Ask yourself, how much time (at a minimum) they need to spend feeding, walking, grooming, etc. for your pet. Add up the hours and then apply a mates-rate minimum wage.