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Most people in the betta keeping community within the aquarium hobby feed them an ultra high protein diet but the available research on the subject actually suggests they do better on a lower protein diet, and when betta diet research is done they generally set the diets they're evaluating at a low protein level in line with that research. I am only aware of two direct studies though, so the support for the consensus isnt exactly super high. I initially hypothesized that it may be because bettas are domesticated, but it turns out research on a couple of related gourami species also did better on lower protein diets, so I'm not sure if thats a sound hypothesis, but I dunno if gourami are considered as much of carnivores as betta, and how comparable they are in terms of diet, as related species can vary a lot. I don't even know that its totally accurate betta are carnivores (when you trace the origin of the claim on Wikipedia that they're insectivores it goes back to a children's book on caring for betta that has crap reviews 😅 I wanna look at research at some point)

Betta are some of the earliest domesticated fish, having been domesticated about a thousand years ago. I would guess that koi, goldfish, and medaka rice fish would also be up their with them. Conjecture on my part, I haven't checked, but I'd guess ornamental carp like goldfish and koi have the longest domesticated lineages

Gourami are actually also related to archer fish, who spit a gout of water at insects on leaves over the water's surface to knock them in, and some people have gotten their gourami to hunt the same way in their home aquarium which is super cool!

Parrot fish use beaks to eat corral and poop out sand, playing a major role in beach formation, and they sleep in a huge bubble of snot. If I recal correctly we still don't know how they get the snot around themselves, as it comes out of their mouths and somehow envelops them moving from front to back. If two part fish are sleeping next to eachother they will share a snot bubble together which is cute as fuck. (I can't find the pic of them sleeping together but its in the video on parrot fish by Octopus Lady on YouTube, which you should totally watch. Parrot fish are REALLY fuckin weird lol)

Pea puffers are tiny adorable freshwater pufferfish, and to my knowledge the only freshwater pufferfish kept in home aquaria that dont have beaks that need to be ground down by eating creatures with shells like clams or snails. Puffer fish have a reputation for being super curious and friendly with their owners, but are fearsome predators. Interestingly it seems like its often carnivorous fish that are regarded as highly personable and seemingly intelligent within the aquarium hobby. Oscars being the classic example of a fish with a "water puppy" personality (shame they get fucking massive 🥲). Goldfish are kinda the only major exception I can think of (this is just an observation on my part, not supported by anything other than "it seems that way to me")

Also just a thing thats super cool about the research on blue streak cleaner wrasse: in the mirror test they put a mark on an animal where it can't normally see it, and then if the animals eventually interact with the mark it shows they have come to understand its them in the mirror, and that the mark they're seeing is something they can interact with on their own body. When animals pass the mirror test they tend to go through stages as they move from being confused to understanding what a mirror is. First they're confused, and often try to fight their reflection, then they start behaving unusually in front of the mirror to try and test if their reflection will always do the same unusual behavior, and then they come to understand what a mirror is, and they start to interact with the mark on their body, having learned its there from their reflection- with the cleaner wrasse they tested whether the mirror would always show their unusual behavior by swimming awkwardly upside down, darting towards the mirror and then turning away at the last second, and PICKING UP A DEAD SHRIMP AND REPEATEDLY DROPPING IT TO WATCH IT SINK IN THE REFLECTION, which is SO COOL. You can see them doing it in the video I linked in my comment on their intelligence testing, and its kinda rad to watch a fish test their hypothesis on what a mirror is in real time using props, not just their body.

Some other neat stuff with cleaner wrasse: they're migratory fish that work in complex variable social groups to clean parasites off of larger fish. They will form cleaning stations, and fish will come to them to be cleaned, identifying them by their horizontal stripes, and a dance the cleaner wrasse do where they wiggle their butts up and down (love an animal that dances to communicate ❤️), and then the larger aquatic creature they're cleaning, called a CLIENT which is adorable, will assume a pose giving the cleaner fish access to their gills and other places there may be parasites, and the wrasse will work together to clean them. In discussion I heard someone say that cleaner wrasse will sometimes eat the protective slime coat off of fish when they are alone, but when other cleaner wrasse are with them they wont because the others will punish them, which suggests they may experience shame or have a sense of socially appropriate vs inappropriate behavior, but I haven't verified that fact with any reputable sources yet so take it with a grain of salt

Last one isnt about fish, but jumping spiders. Researchers have observed that when sleeping jumping spiders exhibit rapid eye movement and leg twitches while they sleep, raising questions about whether its possible jumping spiders could have dreams. We have no idea what's going on in their unbelievably tiny brains, but they are wildly more intelligent than seems like it should be possible for their size and the size of their brains, but they exhibit the signs of what we consider REM sleep in humans, which is pretty crazy (also, we could tell their eyes are moving while sleeping because they used juvenile jumping spiders who still had translucent bodies for the research, and jumping spiders have fascinatingly weird internal eye stalk tube things. So the front lens of the eye doesnt move, but the part at the back of the eye tube that light gets received by does move. The video of their eyes moving while sleeping is pretty funky lol)

Actually here's a fun aquatic plant fact too. People in the planted aquarium hobby often say that terrestrial plants grown "immersed" with their roots in the water and leaves above it prefer to feed on amonia, helping keep fish safe, implying aquatic plants prefer less toxic nitrates (or nitrites?) as their nitrogen food source- research actually shows the exact opposite! Aquatic submerged growth highly prefers to take up amonia, and then if I understand right converts it into ammonium for use. Immersed growth plants (and I believe also floating plants) generally prefer nitrates/nitrites (I can't remember which one, I'll check the book I'm reading that talks about this. I think it might be both) meaning they contribute differently to keeping fish safe, kinda protecting them from a differen stage in the nitrogen cycle (which in home aquaria can generally be simplified as waste starting as amonia, then being broken down by nitrifiying bacteria into nitrites, then nitrates, getting less toxic as you go. Though in reality much nitrification is done by "commamox" or complete-amonia-oxidizer bacteria, straight from ammonia to nitrates skipping over nitrites, or by archea rather than bacteria)

I hope you have a lovely day! ❤️