Depends on what kind of fish. We have little bottom feeders and the food you buy for them is meant to immediately sink to to the bottom. If you feed flakes, sure, they float, but many of the better types of food don't hang around the top for more than a few seconds.

No, that's not what stupid dogs do. It's part of nature, and also bad training. Don't blame the dog, blame your brother.

Dogs are predators. We may have dumbed down the instincts of many breeds over the years, but they are still predators. Look up the term "prey drive". Many of our breeds today still have what is called a high prey drive. I have a Bluetick Coonhound and a Standard Xoloitzcuintli. The Bluetick's prey drive is strong, but I would daresay my Xolo's prey drive is stronger. Hounds are bred for it, but Xolos retain it from when they developed on their own in the jungles of Mexico and our Xolo is just higher strung than our hound. Terriers are also known for having insane prey drives. They were/are bred to hunt vermin. They make great ratters which is why so many horse people love them. Take them out to the barn and they will do their damnedest to hunt any rodents getting into the feed. Prey drive can be controlled, but it take a lot of work and is not a guarantee your dog's instincts will never take control when a small animal zips under its nose. Certain breeds are just less ideal for the home with small critters.

Very true. Some species even do it to wear down their teeth so they don't overgrow.