A Multi-fandom Shit Storm

So disclaimer this is not really properly scientific, just plausible enough fantasy in my mind/thought process.

I wanted them to look similar but i also had different biomes in mind when designing each, so that kind of turned into something like "adapted for different environments even though they hatched from the same clutch". They both share many design elements like the eye motifs, general fin placement, etc. but the way they developed is what varies.

Izuna is built for speed and charm, Madara for defense and power.

Izuna hunts in the giant coral reefs in the open seas and regularly targets divers/ships. Madara hunts in the dark depths, targeting whatever untold monstrosities live down there. Both prefer a warmer environment, with Madara relying on deep hydrothermal vents to combat the colder deep waters.

As such, Izuna has brighter/tropical coloration, though combined with his blue body, when he moves he kind of "blurs", making him hard to detect and track when in motion. When he swims slowly his fins unfurl, and especially with the two translucent pectoral fins he almost looks like an underwater flame. He's got a good grasp of human tongue even though he doesn't speak it, and possesses what the humans call a "siren song". Highly curious, he sometimes gets himself into trouble, but eventually his brother is there to help out (see: kill/eat the problem).

Madara is a (mostly silent) living underwater light show, with his glow serving to lure, warn, and stun depending on how he activates it. Much of the pigmentation in his body is thermochromatic, so in the hydrothermal vents he becomes very vibrant. He also has powerful dark vision, which sometimes grants him an edge over some of the blind populace of the deep. When he swims upwards and back home his colors slowly fade out into a monochromatic grey palette, which kind of makes him stick out there. No matter, he's only there to sleep and see Izuna anyway.

Madara only ventures to the depths during waking hours, the brighter coral waters-while not suited for hunting-are much safer to retire to.

Variations in adaptation:

Fins

Izuna's fins are smaller and more ornamental, when he swims fast they just end up sticking to his tail, reducing the drag as he zooms around. Madara's on the other hand are much larger and translucent, reflecting light and making him appear a little bigger. They also help resist underwater currents like lil' parachutes.

Dorsal fins

the dorsal fins in Izuna's case drape on his tail and sway hypnotically as he swims around, and on his back become a vibrant dorsal fin that acts kind of as a sail, making him incredibly agile on tight turns. In Madara's case these fins developed into barbed stingers that stand upright on his back. He can still control them to an extent and kind of lean them flush against the tail when at ease, but they are mostly stiff and no longer fin-like. Like his stinger these barbs are venomous, injecting a paralyzing agent.

Antennae

Both have the sea-slug inspired antennae for sensing things under-water. Everything from turbulence, currents, and even supernatural energies. The two are equally sensitive, but Izuna's is shorter, since he's constantly zooming around the shorter length both increases their efficacy and prevents them from getting snagged or tangled. Madara's longer antennae float away from his head and add to the intimidation factor, they also appear as eyes "behind his head" so anything approaching from the back will think it's being watched. (while many of the deep sea dwellers are blind, some of the more dangerous ones are not.)

Tails and Stinger

Izuna's stingers and tail are separate, in his case the filaments on his sides are what developed into stingers. They make close-quarters attacks easier and can also be wielded by him directly as daggers, though he also can use them to sneak attack while charming a target (can be controlled independently). In Madara's case one of the two tail-fins developed into a powerful serrated stinger, useful for lethal piercing and hacking attacks.

Both are incredibly venomous, with the paralytic venom powerful enough to kill a grown man in under 15 minutes. Both also have a false eye on the tail, capable of mesmerizing/hypnosis. in Madara's case, the remaining tail fin developed an additional large bioluminescent eyespot that can flash-stun prey.