I think all fish are really omnivores... with tendencies towards carnivore or herbivore...

[Magnum Man]

the local pet store had been out of frozen foods, when I have gone in lately ( which hasn't been regularly ) stopped by yesterday, and was able to resupply on brine shrimp, and mysis shrimp... this morning all the tanks got a good mix of shrimp, and blood worms, and everyone eats them, including my plecos, and bitterings, and I have watched my cichlids, and bichir eat algae cookies... I know the digestive systems are different, but I think everyone eats at least a little of everything... I would hope offering them a variety is good for them... at least in a mixed species tank... maybe in a species only tank, one or the other could be fed, but you would think they might mouth, but not eat the wrong foods, and they may choose mostly their own kind of food, but still choose to eat some of the other... thoughts???

you may be a meat and potato kinda guy, but a little salad is not going to kill you once in a while...

 

[Lcc86]

I recently witnessed my betta eating a bit of algae wafer that I put in for my snail. Looked it up and saw it's not really ideal for them as it can cause bloat so I made sure not to feed him for a couple of days after that!

 

[gwand]

Like humans, fish don’t always make good food choices. In my park, ducks and koi will eat bread crumbs that well meaning people feed them. Not a good choice for either species.

 

[Magnum Man]

a couple hours later, I saw the little panda garra in the bitterling tank, "hoovering" the sand bottom for brine shrimp... typically they are on the plant roots...

 

[Back in the fold]

Fish are opportunists . They eat whatever they find in case nothing else shows up that day . You’ll eat anything once you get hungry enough .

 

[emeraldking]

I think all fish are really omnivores... with tendencies towards carnivore or herbivore...​

Well, there is a difference in teeth in typical carnivores and herbivores.
But yes, there are omnivores that lean more to carnivores and omnivore that lean more to herbivore.

 

[GaryE]

What goes in the mouth is one thing. What they can successfully digest is another.

Teeth are different, stomachs are different, intestines are certainly different. Give the same food to an insectivore or a herbivore and what nutrition they get from it will be different. They're like us - we'll eat almost anything, but we'll thrive on some foods more than others.

From breeding fish, I can tell you a fish not given foods that correspond to how its body works is rarely going to be healthy enough to have young.

 

[Magnum Man]

maybe brine shrimp are just irresistible??? even to a salad muncher

 

[GaryE]

Most of my herbivores eat brine shrimp, although most of them aren't as efficient at catching them as predatory fish are. It really depends on their evolution. But again, I can eat big macs followed by cheesecake 3 meals a day, every day, but it'll kill me as I'm not adapted to handle fat long term.

I think the cory group, for example, find that food frustrating, even if they are insect larvae eaters, because they aren't adapted to get them til they settle on the bottom. By then, from our point of view, the baby brine shrimp are less nutritional. But it remains possible they are still good food, compared to processed flake or pellets. My corys eventually do eat them. They eat inverts in the wild. But they like worms better than bbs.

A lot of algae eaters don't prioritize algae. Mollies can be seen in the wild picking at underwater cliffs covered in algae strands, but they are hunting what lives in the algae, and taking both types of food in.

Fish like Farlowella or a lot of Loracarids can die without plant matter. Beyond instinct and nutrition, a lack of plant matter leaves them without enough fiber for their guts to work, and they can bloat.

With some of the herbivorous livebearers I used to keep, ones that provided nutrition to their young internally as mammals do, high protein food was reputed to be fatal to both fry and the mother, as the fry would be too large to be dropped. Plus without plant food, bloat was always a risk, from intestinal blockages/low fiber.

So I'd say your idea is wrong. There are too many variables, and too many weird fish and weirder natural histories out there to generalize like that.