Yes I’m sure there’s already a lot of suggestions about this.

Sacabambaspis

It would appear in cave biomes and would be Uncommon. Offsetting the chances of fishing a blindcat. When you think you’ve got another blindcat, it’s this jolly little fellow who just wants to invite you to his underwater cave for algae slushies.

Ribbon Eel

Found exclusively in coral reefs, this fish is related to the other eel mobs but it is more docile towards players. Rare rarity.

Stingray

Comes in several colours (based on real life species), found in tropical, coastal and oceanic environments. Varieties include pelagic stingray, bluespotted ribbontail ray, leopard whipray, ocellate river stingray, mangrove whipray and giant freshwater stingray. Rare rarity.

Seahorse

Also have many colours. Spawn exclusively in coral reefs. Uncommon rarity.

Sea Angel

A kind of sea slug. Found in frozen oceans, rare rarity.

Blue Galucus

Another kind of sea slug. Shoreline region, rare rarity.

Carp

We have koi. The carp would be found everywhere in aboveground freshwater. Common rarity.

Lungfish

Rare fish found in arid regions.

Electric Eel

Found in the rivers in tropical regions, this fish can generate electricity and hunt by electrocuting prey. Rare rarity.

Candiru

A tiny parasitic catfish in tropical waters, be careful, you’ve heard the stories. Rare rarity.

Arapaima

A large fish from the jungles. Epic rarity.

Crocodile Icefish

Exclusively found in deep frozen oceans, this fish would be Epic rarity with its transparent body, blood, and visible internal organs.

Nether Fishes

I hope it’s possible. The words in the parentheses are the names I propose for the fish.

Ammonites (Vibrant Shells)

They can be found in all nether biomes and come in many colours and shapes. The tropical fishes of the nether. Common rarity.

Ichthyosaur (Magma Lizard)

Also very colourful, dolphin of the Nether. Common.

Opabinia (Crooked Shrimp)

With five orange compound eyes, large raptorial claw and a teal body, this creature is one of the many odd species of the Warped lava oceans. Common rarity.

Anomalocaris (Contorted Prawn)

A teal and purple shrimp looking creature with two large barbed raptorial appendages and a segmented body. Rarely found in the Warped lava oceans.

Dunkleosteus (Obsidian Jaws)

With obsidian plating on its face and jaws, replacing teeth, this fish is only rivalled in hunting prowess by its golden counterpart. Epic rarity.

Helicoprion (Golden Sawblade)

This shark like species has golden sawblades in place of teeth. But don’t let this fool you, these blades are not weak like gold but are reinforced. Highly sought after by Piglins for tool making. Epic rarity.

Horseshoe Crab (Ancient Blood)

This creature is related to the Overworld’s spiders and cave spiders, but looks almost nothing like them. With its tough brown exoskeleton, it is nearly invincible. It is commonly found in the Basalt Deltas.

Eurypterid (Netherite Fossil)

A relative of Ancient Blood, it is far larger and has large grasping arms to grasp prey. Its exoskeleton is made of netherite and is nearly impenetrable. Rarely found in the Basalt Deltas.

Kraken

Perhaps there is a reason why the Piglins haven’t ventured over the lava oceans. The Kraken is believed to have descended from squid in the Overworld. Having evolved in the lava oceans, when brave adventurers stopped sailing the nether oceans, they reduced in size due to a decreased food supply. Rare rarity.

Related Features

Volcanic Glass

Due to the opacity of lava, building aquariums for nether fish would be difficult. So I suggest a new glass type, volcanic glass, a glass type that can reveal what hides in the lava.

Goldfish Breeds

Another thing I’d like to suggest is more goldfish varieties. Goldfish have many breeds and it’s a shame we only have one.

Schooling

Also it would be nice to see schooling behaviour in certain fish species such as butterflyfish, damselfish, angelfish, tuna, salmon, cod, anchovies, and clownfish.