I dunno Article is full of flawed assumptions: "who has ever seen a guppy eat leaves", "they probably eat insects so are carnivores"
Guppies will eat worms, if there's no worms they'll eat aquatic crustaceans, and if there's not for those they'll eat terrestrial insects which pose greater risk: birds.
If there's no aquatic crustaceans they'll eat protozoans and phytoplankton off leaves. I have notice if you have a well planted tank and it's rich in zooplankton which such tanks often are, to the point where Vorticella are visible and if the tank is big enough and number of fish small enough, you don't need to feed them. This has been observed in at least two killifish species.
Buppies are too smart to eat the leaves, that's where their food grows and just like puffers won't kill all the snails (they farm a few and know better than to kill them all) then they won't eat the leaves. Unless there is no other source of food and then they will. Anyone that's kept anu Poecilia species nows all they do all day is pick at every surface imaginable.
Scientists that study guppies make a point of giving them vegetable matter as they don't do as well without it. This point isn't arguable. As far back as pre WWI paste foods made from oats, liver and spinach were used successfully. More successfully than without: keep in mind in that era a) molecular biology and biochemistry were founded base don the dicovert by 1931 the natroir eo fhr chemifcwl bond b) by 1933 substanced wre idendieified withpit with life is impossible and are rwuire din tumy amounts: copper, managanese, sleenoum are exemaples of some ehcmwca eemtns, while more complex biolgival compoints keep neing socvwted. Vitamin A was original, called "vitamin", as it ws thought this was the magic substance without whcih life is nt possible, until another was found, this vitamin A and B. By 1933 the biolgical mieold L-Ascorbte jimuned with "C". There are now at elast a dozem B vitmains and by 1957 selenium was added to the list the last one to do so. Previusly it was regarded as a poison, so most literitir eo niit is worng.
At the stake these fish are any buf they eat may be up to 50% phytochemicals, tiny flies covered in pollen plus the high potency food they eat is in their gut. ot much nutrition in bugs, it's the guts that matter.
Just as in plants where macronutrients are N, P, K and "iron + trace" are the micronutrients then so too ar fats and proteins animals macronutrients and hie "Fe + trace" has a different meaning.
Without the minuscule amounts of various enzymes, minerals and complex botanical chemicals in plants no animal would be able ot exists, they're essentiual to lfe. Thus not as many pure carnivores are in actual fact obligate or exclusive carnivors. Examples do niexist: the Masai of the African veldt can live on blood and milk. The people of the polar region can live on fish, seals and walrus and whales and nothing more.