Hello, I have found a few things with the 3.0.0-1.20 version that may warrant some fixes, and a couple things that I thought could be added to. Here's what I've got so far:

Lack of config. Several of the things in your mod could benefit from user tweaking and so I suggest a config with spawn rates for the fish, spawn density for the structures, enable/disable certain features (fish/structures/potion effect/specific items, etc.)

Blazefish do not spawn around the nether fortress pieces. This has been recorded in another issue here. I would also say that lava underneath actual nether fortresses should also spawn the fish if possible, or that the pieces generate more often by default

Lava sponges appear to not be spawning, and the /locate structure command can't find them, so I'm unsure where the are meant to be obtained. I think they could potentially be added to the lava fishing treasure loot table (Along with the Eyeball Fish Eye and Nether Fortress Grouper Plate which arguably should not be considered fishing junk)

Soul Sucker Boots have an absurdly high defense but not much else going for them, making them so unbalanced it should probably be changed. I recommend that - They are treated as gold by piglins, come automatically enchanted with soul speed 3, and have a defense similar to iron. When on soul sand/soul soil - the player is unable to jump but has max knockback resistance and a slight regen effect as the boots grip on and feed from the souls in the terrain.

Fishing with the Lava Fishing Rod spawns the fish as an entity instead of an item. That is different from vanilla mechanics, but I think it's actually much better than how vanilla does it, however I suggest that the fish are temporarily stunned for a few seconds after being hooked so the player can actually catch/kill them and also that this is able to be toggled via config. Also, the fish rates are very skewed, if possible maybe the rates could reflect which biome the player is fishing in? I can't tell if this is already implemented so just ignore if it is.

Many nether fish seem to not have a use/have very limited use, so I figured I would throw in some suggestions.
Lava Pufferfish have two good uses currently and don't need a change, but potentially lava vison could provide fire resistance as part of its effect in order to streamline the player experience. Alternatively, lava vision could be acquired from Eyeball fish instead, and Lava Pufferfish could be used to brew potions of wither to be used in combat.
Obsidianfish could be used to craft crying obsidian as well, but both blocks can be acquired faster by mining/piglin trading. To stand out, I think the obsidianfish could be used to brew a potion which gives knockback resistance and immunity to environmental hazards (lava, fire, magma blocks, sweet berry bushes, cactus). I also think the obsidianfish should be able to survive in both lava and water, if possible.
Searing Cod have potential for being used as furnace fuel, rather than dropping coal.
Bonefish, rather than being used as bonemeal, could be right clicked into farmland to create a block which grows crops twice as fast, preferably using tags so other modded crops can be grown.
Wither Bonefish could be used or crafted into an item which the player can right click on any small flower to turn it into a wither rose.
Blazefish could be used to craft an item or potion which wards off blazes and stops them from targeting the player.
Magma Cube Fish could be fed to frogs (via the item, not the fish) to get froglights. Probably easiest to just add a right click functionality to the item
Glowdine could be used to give the player easier access to the glowing effect via brewing
Soul Suckers could lower the players max health over time when in a certain radius, returning after the player gets far enough away
Fortress Groupers at present already have a use by dropping plates, but the item itself doesn't have any use. They could either be crafted into nether bricks, or potentially used in an item that finds nether fortresses.
Eyeball fish seemed like a better candidate for Lava Vision as stated before, but an alternative use could be to craft an item (rather than a potion) that has the same effect. Some sort of lava goggles that could go in the helmet slot (or goggles slot if curios/trinkets is installed).

The mods adds a tag called "Nether Salad Foods" which is completely unused but has a lot of potential. All the fish could be prepared into meals as well as the items in the tag.

It would be interesting to see some kind of difference under the surface between the biomes. As of right now, the basalt deltas are very identifiable because they just generate like that, but potentially the Warped Forest could have a higher density of Warped Kelp and some really messed up looking coral. Crimson Forests could have crimson variants of the kelp/seagrass, and maybe even a hostile fish like a shark of some sort (which could drops scales/teeth to craft armor which gives a better movement bonus in lava/increased damage in lava), and Nether Wastes could contain blackstone shipwrecks with Bastion loot in them. And of course, any more kinds of corals/plant life would go a long way as the nether is naturally super empty

Overall this is a very promising mod and I appreciate the work and creativity that went into it. I did notice that 3.0.1-1.20 has two Forge releases on the same day but no Fabric release, I hope Fabric is still a possibility for you!

Hello, thanks for all the suggestions, I do appreciate it.
I can add some configs for the fish spawning. The features however can be changed with their corresponding file under worldgen -> configured_feature or worldgen -> placed_feature. placed_feature being the one that can be used to change how much it will spawn in the world.

The issue with the blazefish has plagued me for a while, since I can't get them to spawn that good, I have seen them spawn but not a whole lot. I will take a look into it for the next update.

Lava sponges are a feature, so they cant be located, however they are pretty rare. I could probably up the rate at which they spawn.

The issue with the soul sucker boots having a high defense (not intended) was fixed in the 3.0.1 update for fabric (I accidentally tagged it with forge).

The idea for the fish being stunned for a bit is a good one and I will implement this for the next update. Right now they can also be turned in to items with the config file.

I shall take your ideas for the fish in consideration, for the next update as they are pretty decent ideas.
The magma cube fish however, can already be fed to frogs.
The fortress grouper plate and the eyeball are used to craft lava glass. which allows you to see in the lava.

The nether salad foods is a tag added for compatibility with farmers delig, however since it wasn't updated yet it is unused.

The ideas for the difference under the biomes are also good and I will take a look at implementing some and perhaps some other ideas.

The two releases for 3.0.1 were one for forge and one for fabric. I seem to have put the fabric one with a forge tag.

Hey thanks for reading through the suggestions! I had kind of figured that 3.0.1 might be tagged wrong but wasn't sure.

I didn't know magma fish could be fed to frogs already! That's a great use for them.
The fortress grouper and eyeball fish suggestions were made because while the entities can be killed to drop the items used for lava glass, the fish items you get from them are basically without use. I figured turning them into something that can be used would be most effective at bringing some interest, but another route I could see is doing something similar to the soul suckers. Use the nether grouper item, craft it into plates, use the eyeball fish item, craft it into eyes, and then remove those drops from the entities. I may end up doing that as a datapack for now just so I don't feel bad throwing items away lol
Lava glass, by the way, is SUCH a cool block, I absolutely love the concept.

The tag makes more sense with that in mind, I figured it might have been added for other mods, but why not add in some of your own items? Your art is good, and it would help the mod feel like a more complete package that can integrate with food mods, rather than relying on them to add those items.

Again, thanks for reading through everything, I know that was quite the list, I haven't seen a mod with this kind of idea but it just makes so much sense and I will definitely be watching to see where it goes!

I do like the suggestions and have added a lot of them ^^
If you got some more do write ^^
You can also send me a dm on discord

I wrote a lot of those suggestions basing on the content that was already in the mod, but I didn't really touch on anything added since that list was long enough lol, I can share a few though!

One of the things I think the mod could use is variety, not in the amount of fish but in the types of fish (cuz you already did a great job in the amount of fish!). I think to start, a general guide could be to add one new type of fish to every biome in the nether, for instance:

• In soul sand valleys, a new ghast inspired octopus spawns. It's neutral but if attacked it will wrap around the player, causing damage, until the player manages to deal enough damage to cause it to flee. When killed, it can drop tentacles which allow the player to cling to walls and ceilings like a spider, or, the tentacles could be used like a ranged lead, extending to grab entities and then attach to them like a lead. The latter option would have more practical use I think
• In Crimson Forests, you could add a hostile fish type, this time shark inspired. While a more classic shark design would work well, you could also make an ambush type predator that lies flat on the sea floor and attacks mobs above it by quickly lunging and latching on. It could drop serrated teeth which could craft a new kind of trident, when thrown and hitting an entity, instead of coming back right away it could stick into the mob and deal damage over time, lasting until the mob dies or 5 seconds have passed. You might have some trouble with the technical side of that, in the cases of entities that disappear or something like that, but I think you could handle those as they come up
• In Warped Forests, since the biome is supposed to be mostly empty and kinda creepy, some kind of alien looking eel which will always dart away from the player, but if they manage to get the drops they'll receive eel skin which can be made into armor which allows players to swim in lava like they do water and gives a decent movement speed buff while doing so. There already exists a mod called Hell Strider I believe which is Depths Strider for lava, it would go with this suggestion perfectly
• Finally, since the Nether Wastes are usually where the giant lava oceans are I was thinking something big... like a whale. Now, this creature can't be fished up but you can fish up nether ambergris, which could be a fun item to give uses for. Traditionally ambergris is used for perfume, so you could make it super rare and craft a potion that makes the player essentially invisible to all mobs in the nether. You could also make it craft into netherite scraps, giving some incentive to get Luck of the Sea on your fishing rod and get to fishing. The whale itself probably should drop anything too useful, maybe something generic like whale meat, but while alive the whale will occasionally drop ambergris (about the same rate as Sniffers find items).

Some other things I was thinking of was maybe an entirely new biome, so to extend on one of the suggestions I already made, the nether reefs!

• The nether reefs would generate similar to the large open lakes already in the nether but they would generate in such a way that the bottom of the biome is basically one block away from bedrock at all times, so the biome is always deep enough for interesting features and builds
• It wouldn't be a reef without some coral, and I'm thinking you could make some interesting visuals with colors inspired by bone, wither, gold, and iron. Real life corals build using minerals, so having the corals in the nether build using the minerals from bones, gold and iron would be an interesting translation. Of course, they would only resemble the colors of those materials, they wouldn't have any functional use (keeping in line with the vanilla corals)
• The reefs would be home to several unique mobs, like sea horses, which have colors of red, orange and yellow, and their texture pulses like lava/magma blocks. While they don't have any uses on their own, having them in the lava within 5 blocks of where the player is fishing will boost the rate that fish bite as they're a tasty snack to most of the fish in the lava seas
• Another mob found only in the reefs would be the plated turtle, which has a shell made of netherite, and is functionally invincible. Because they can't be killed, they don't have any drops, but if you feed them some nether sea grass they will start heading towards the nearest debris cache (will cover that in a second). I think adding mobs that have uses other than just drops is super cool and would definitely suggest brainstorming up some more!
• Finally, occasionally you will find an open section of the reef with no corals, the seafloor being made of soul sand rather than the traditional netherrack. All the fish that spawn in the seas should actively avoid this area in order to create some mystique as well. Found in the center of that area is an active, albeit very run down and malfunctioning, netherite construct. This new mob will pop up from a block kind of like a Shulker, and will shoot at anything within a 7 block radius with a beam like Guardians, however, it shoots a bit faster and pushes entities farther away than Guardian beams. Killing it yields some scraps, and more questions than answers. While this is not very in line with "fish" concept, it is very in line with the "fleshing out the nether depths" idea, so I figured I would throw it in here as well. New structures and such that expand the unwritten, broken story of Minecraft is something I think fits perfectly with that mission!

Finally, some odds and ends which would make things a bit nicer/flow together a bit more:

• Netherrack sand for the ocean floors, just something to clearly stand out from the vanilla nether oceans by adding a single block
• Unique structures, I know I recommended ships like the vanilla ocean has, but I think it would be even better if they stood out. There's a datapack that turns end ships into air ships, so I was thinking maybe the nether ships are more like submarines! You could also hide treasures like ancient debris caches, where some long gone race was carrying cargo and over time the lava melted it into the debris we see today. You could also make structures that float on the surface of the lava, like abandoned nether wood rafts, carrying woods and food that some long gone traveler never got to use (bonus points, spawn a wither skeleton on board for a traveler not so long gone).
• I already suggested more food items based on the fish but you could also add more plants that drop items you can use for food. Some weird alien fruit that grows on the ocean floor, or a spread of vines that can only grow in open ocean and spreads into a sheet that can be stood on, but also harvested and eaten. Possibilities there are endless, but anything that stands out from what already grows in the overworld would make this mod really pop
• Simple, small deposits, like boulders of blackstone, obsidian, smooth basalt, just something to break up the flatter terrain.
• Speaking of terrain, if possible, terrain generation changes that make the lava oceans generate more similarly to actual oceans, with shallows, dropoffs, under sea vents, etc, but that seems like a LOT of work so definitely not an easy one to pull off

Thanks for the suggestions, I will take a look at them and might consider some of them.
Some things like the sand for the ocean floors is not really that doable since that would mean overwriting the nether json files, which I can do, but if someone also does it, its just a matter of which one gets chosen so I want to avoid stuff like that unless there is a good way to do it.
As for the swimming in lava, there is the hell strider enchant the mod adds that could help with this.

In that case maybe the sand is best suited for biomes you add yourself then!
And oops, I hadn't checked the mod in a bit and forgot you already also add that enchant, my bad 😅 I had been looking through other mods and seen that it was also a standalone, so that's what popped in my mind first

The latest update adds some of your suggestions ^^ aswell as some fixes.

Great update! Adding the crimson variant of the plants and making sure all the fish have functioning uses adds a good amount of polish. In my pack, I use a couple mods that give descriptions for things like enchantments and potions, I see that you've included a description for Hell Strider which is great! I was thinking, although "Lava Vision" is obviously pretty self explanatory, it may help to give a description just for the sake of polish as well. Something like "Allows the affected to see clearly while under the surface of lava"

I also noticed the Eyeball fish entity doesn't have a translation, there's a minor typo in the advancement for crafting a lava fishing rod, says yourney rather than journey

I see a bunch of translations for food items, those look like a lot of fun and could be in the base mod! Rather than adding the items, the compat could switch the recipes over once Farmer's Delight is loaded. I was trying hard not to add Farmer's Delight to my pack but if you think the foods belong compat only I might have to reconsider... 😅

Also, with the glowing potion, I was wondering if you could rename some things. It actually is due to the arrows you can craft with the potions being functionally identical to Spectral arrows. I think you could potentially rename the potions to "Spectral Potions", and change the recipe to be more similar to the vanilla Spectral arrows. Rather than use 8 arrows, they should use 4 and give 8 to keep in line with how normal Spectral arrows double the base arrow in the recipe (that, or you could have 8 arrows craft into 16). Then, since they're basically the same, have the base glowing potion craft vanilla Spectral arrows, and have the stronger glowing potion craft "Enhanced Spectral Arrows" - you could add this as a new item or just retexture and rename the potion arrows that are already there. Let me know what you think!

I have fixed the typos and added the eyeball fish translation