Why do some fish have tall flat bodies?

Freshwater fish like angelfish and discus have their slim and elongated bodies because it just makes it easier to navigate their surroundings. During the rainy season in the Amazon, the Amazon river floods into the surrounding forest. The river becomes a tangle of sticks and leaves, very hard for bigger, longer fish to navigate. The angelfish and discus's flat and short body shape allows them to take full advantage of this. Being flat, they can easily slip through very thin gaps in between plants and sticks to get to the deeper parts of the forest where there are almost no predators. As you mentioned, why can't they just be proportionally thin and long, being short from head to tail allows them to corner very well. If they end up in a thick bunch of branches, they can turn on a dime to navigate the branches where other longer fish would get their tails stuck. It's just another evolutionary advantage to allow them to take full advantage of the flooded forest's protection. When they get very far into the forest away from predators, they lay their eggs and raise them until the waters recede and they need to return to the main river. Also, fish can't breastfeed because they aren't mammals and have no mammary glands (breasts).

As for saltwater fish, they are slim primarily to hide from predators. Many deep sea fish like hatchetfish are thin so that they can't be seen against the lighter surface waters by predators beneath, or so that they can sneak up on prey from beneath without being spotted. Other shallow water fish like tangs are flat so that they can easily navigate the reefs they make their home, similar to the angelfish and discus of the Amazon. Their slim body also allows them to hide in small cracks or caves in the reef. The reason why they aren't proportionally long is so that they can fit in shallower cracks or crevices that longer fish couldn't.

There are some long and thin fish both salt and freshwater, like the ribbon moray eel, the pipefish, the oarfish, the silver arawana, and the knifefish that are proportionally thin and long, not so short like angelfish and hatchetfish. These fish are all adapted for specific purposes, and obviously evolution got it right otherwise they wouldn't still exist. Hope that answers your question.

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Flat Face Fish

Well, wild angelfish can easily blend in with tall plants. The stripes on their bodies, and the tall shapes make them difficult the spot when in tall, thin vegetation. They wait for prey to come along, and grab it with a quick burst of speed.

Marine fish like Atlantic moonfish and similar species, tend to be silver. Their shape is for the same reason. Since many predators hunt from below or above, the slender shape from above makes them difficult to spot. The silvery color helps them blend in with the water. When they travel in a shoal, they can confuse predators and make it difficult to single one out. And if they turn to face or flee, the disc like shape helps break up their form more.

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