As for happy and unhappy. I’ve been doing this for over forty years now and I can certainly spot an unhappy fish or a happy fish, so yes fish have the capacity to be happy, albeit that is most probably not in the same way humans do.
I've been observing fish in the wild, and in glass boxes, for 50+ years, and one thing that I have observed over that timespan is that there have been many myths & misconceptions repeated by the masses over those years. Such as protein causing bloat, something that I wrote about in great detail many years ago, one of those old posts was later made a sticky here on MFK.
Bloat - Causes - Cures - and BIG Myths | MonsterFishKeepers.com
Yet this was the belief for decades, with many "experts" stating the same.
Many members here believed, and no doubt some still do, that eating wood is part of a nutritional requirement of plecos that rasp on wood in the wild, as though they would die without it. lol They don't. Which is why I posted the following close to a decade ago, and it too was made into a sticky.
Can Plecos Digest Wood? | MonsterFishKeepers.com
I also wrote the following many years ago, and IMO the exact same thing applies to "wood eating" plecos.
In the wild, the cichlids found in the Rift Lakes have evolved & adapted to living in certain niches of the lake, which over time has forced them to become specialized feeders. (as per Ad Konings) Yet all of these specialized feeders will readily eat anything that's available. (as per Ad Konings) While a fish classified as a strict herbivore (such as a Tropheus moorii) may indeed spend its entire day scraping the aufwuchs, I can assure you that they would much rather eat a handful of worms if given the opportunity. In the wild they eat low quality foods because that's the only foods available, not because they choose to!
And while Tropheus and various Mbuna species may in fact be classified by the scientific community as strict herbivores, the reality is that even though algae dominates the stomach contents, the actual foods that make them grow are insect nymphs and larvae, crustaceans, snails, mites, micro-organisms, and zoo plankton, not vegetable matter. (as per Ad Konings)
Their long digestive tracts are designed as such so that in nature they can break down the complex plant matter that they consume, which doesn't mean that they can't properly assimilate more easily digestible forms of protein. Apparently this is a concept that some hobbyists fail to grasp.
Keep in mind that the vast majority of fish are opportunistic feeders, and are all omnivorous to a certain extent. Cichlids classified as carnivores don't just eat meat, any more than a herbivorous cichlid just consumes vegetable matter.
Plecos in the wild don't eat wood because they "enjoy" eating a **** sandwich all day, every day. They do so to survive. This feeding behavior was not created out of a need for enjoyment, or pleasure, or because it makes plecos happy, it was part of an evolutionary trait that was the result of the natural feast or famine cycle that nature forced this genus/species in to. Wood eating plecos have evolved and adapted to survive under conditions where most fish would perish. And if you read the previous scientific papers that were linked to in the other thread on this topic, you will understand that the wood eating behavior is out of the need (in the wild) to utilize & promote certain bacteria in their gut. We cannot assume that this is the same requirement in a glass box, or that the same microbiome is being created in a captive fishes gut, vs those same species in the wild. Sorry, not so simple. I welcome future research on this subject.
BTW - when I used the term "meaty foods", I was referring to higher protein/fat pellets, not market prawns etc. And I can state that I have seen many "wood eating" species of plecos, with LOTS of wood in their tank, go straight for the pellets when they had the opportunity. I have observed that exact behavior, over several decades, and numerous "wood eating" plecos. Gee, I wonder why? Might it be that in captivity, just as in nature, these fish have learned to adapt, to a diet that is nutritionally speaking far superior to gnawing on a chunk of Malaysian bogwood? Hmmmm. Stupid fish?
Seeing as some folks like to anthropomorphize their pet fish, allow me to add this. A fish that has survived the cruelty of feast or famine for thousands of years, and has learned to be
happy with eating a **** sandwich daily, has more than likely also adapted enough over those thousands of years that when a steak dinner falls in their lap, they can very
happy eating that as well. Maybe even more
happy than when eating the former? Stupid fish?
And again, there are safe substitutes to wood for a pleco to rasp on - if one chooses to keep this group of fish without wood. I am in no way attempting to tell anyone that one way is better, just sharing an alternative way to also successfully keep this genus of fish in captivity. Sans wood, they won't starve to death, and they won't die from being un
happy.