My almost 5 year old daughter wants to know what happens to the water in her golfish bowl. She says the fish drinks it. I am not so sure.

Capuchin

Most fish do drink plenty of water. However i'm sure you're aware that what goes in must come out... they excrete most of the water right back into the bowl (their urine is very dilute because of the amount of it that they process). So this wouldnt cause the water level in the bowl to go down.

What you're seeing is evaporation. Just like how puddles dry up.

templelane

Fish drink differently depending on whether they are salt water or freshwater fish.

Marine water bony fish need to drink loads and secrete the excess salt in order to maintain water homeostasis (balance) in their bodies. They have little concentrated urine.

Freshwater bony fish - mostly gain their water from osmosis (passive diffusion of water) across the gills and their food, I don't think they intentionally drink. They have to actively get rid of water so they don't burst so I think this would be unnecessary. They excrete dilute large amounts of urine. They get their salts from their food in order to maintain a balance.

The water will be leaving the bowl by evaporation and she probably thinks she can see it drinking when it gulps air at the surface. This means you might need to get a pump to oxygenate the bowl if it does this a lot. Or possibly she thinks it is drinking when it gulps in water in order to pass it over its gills.

Probably more info than you need but I think it is good to tell kids about random stuff. It's great you're finding out used to really annoy me when adults said 'it's magic' to me as a kid.