I've got experience with this. I went on a 3 week vacation and hired a pet sitter for my fish. He was bonded and insured so I felt safe leaving him the keys. I gave him very clear instructions on what to do. I put pellets in small ziploc bags and told him to only put 1 bag in a tank per feeding and he was supposed to feed my fish 3 times a week. I also told him to absolutely NOT try to clean any algae or change the water inside. About 2 days before I was supposed to come back, I got a very frantic call from him saying that my 100g glass tank had bust open and all the water was on the floor. I didn't get his call though because my phone didn't work overseas.
When I got home, my den area was a mess. All the water in my 100g had seeped under the floors and all the wood was warped. I had to completely throw it away and I replaced it with tile. I kept asking him what happened and he just said the water was all over the floor when he got there one day. I also asked him if he had ever tried to clean the inside and he said no. I didn't try to get the money back from his insurance or anything because I couldn't prove exactly what caused the tank's seam to split. It cost me about $600 to replace the flooring.
I had prepared more than enough food for 3 weeks, but I could tell he overfed all my fish because all the bags were gone. So I think he gave them 2 bags per feeding instead of the 1 bag I had told him to. So let that be a lesson -- sometimes even when you try to idiot-proof things like pet sitting, things can still go wrong. Anytime you leave fish with a person who doesn't know anything about fish, they tend to overfeed them because they think the fish aren't eating enough and might die. Plus they probably think you'll never find out if they feed more than recommended, so why not? And they also think feeding fish is fun and the fish look so happy when they're eating, so it won't hurt to give them some more. Anyway, that whole experience made me realize that fish sitters are a bad idea and to just let fish go hungry if it's only 1 week or use auto feeders if it's longer than that.
