even vegetarians are omnivores really...

[Magnum Man]

or... everyone loves brine shrimp, and blood worms ( shrimp cocktails )
.... in my barb tank algae is often added as prepared foods, along with the tanks bio film ( so these fish aren't starving ) in fact my group of Crossocheilus reticulatus (spot tail algae eaters) are growing rapidly...
today that tank got fed shrimp cocktail, and the spot tails were very aggressive eaters of brine shrimp, and blood worms...

 

[Magnum Man]

biggest spot tail just starting to get the yellow fins of maturity

 

[GaryE]

Vegetarian is a human choice. The guts of a herbivore aren't the best for digesting animal based foods, but deer and squirrels will eat meat on occasion. Herbivorous fish will too. It's just that it leaves them more prone to blockages, as their gut structure needs a lot of roughage.

There's an old trick of giving peas to insect eating Bettas to deal with blockages from a low quality diet. It goes both ways.

 

[CassCats]

I've seen swans eat fish.
Photo I've taken a couple years back

Most things lean more omnivore, and it's crazy thinking of a cute little squirrel going nuts over raiding birds nests for eggs and nestlings. Its actually fairly common for them to do it, especially pregnant squirrels or nursing squirrels.

That said, many of the algae eater family are known omnivores and should have a small portion of protein in their diet. Some are known to pester the slime coat of tankmates if they don't get enough. Chinese algae eaters turn more carnivorous as they mature, which makes them a fish some need to carefully research because they can pose a problem if not cared for properly.

Siamese algae eaters lean more veggie, but still need a small bit of protein too.

Their flying fox and false flying fox relatives are the same way.

This dude, Bruce the Brute, is a ceratogarra cambodgiensis, aka a false flying fox and he's the same deal. Gets fed Bug Bites pleco sticks, goldfish pellets, kelp wafers, and spirulina flakes, but he also gets grocery store shrimp and squished mealworms.

 

[GaryE]

I watched wild mollies, Poecilia mexicana or orri (not sure, but scarlet finned) feeding on the wall of a crystal clear Mexican Cenote. The aufwuchs algae was a few cm thick, and they were eating some of it, but they were also picking through for what was living in the algae. It was beautiful to see in the sunlight, but also kind of interesting as there were hundreds of fish working hard on that underwater cliff wall.

 

[emeraldking]

There are three types of omnivores: One that does eat greens and meat at all times, omnivores that lean more to meat and omnivores that lean more to greens.

 

[emeraldking]

biggest spot tail just starting to get the yellow fins of maturity
View attachment 361117

Beautiful specimens...