Yes of course, i put up that picture above specifically because it illustrates the beautiful juxtaposition of sharp canine teeth ripping into some fibrous vegetation. but the whole point is they can and do subsist entirely on vegetation when they need or want to for whatever reason. After all, being strictly herbivorous would put them at the same disadvantage carnivores face.
I feel like everyones getting wound up over the wrong reasons here. Just as OP shouldnt prescribe their vegan diet plan to their fish we shouldnt be ascribing our warped political beliefs to them either.
Your example is extremely disingenuous. Given the choice Pandas would most likely eat meat (they do but mainly only carrion in the wild as they de-evoled their drive to do so) but they evolved so far down an extremely niche path during the previous Pleistocene (ice age) that they are now at the brink of extinction. Had humans not evolved to what we are today the giant panda species may have evolved into a true herbivore in several thousand more years but as it stand they are a predatory animal forced to be herbivores due to environmental changes. However, even regardless of humans, the most realistic view of giant pandas as a species paint them as an evolutionary dead end.
Still, to humor the point, the only way to replicate such a change in the home aquarium setting is to force feed and breed several hundred generations of fish until you keep getting fry that survive long enough on a vegan diet and carefully interbreed those fries until you create a substrain that will inevitably go extinct due to having such a strict nutritional diet. My judgement/opinion on such matters is immaterial as we already do this type of breeding for aesthetics; however, most people on this forum (and likely any serious fish keeping forum) would likely find this reprehensible and condemn it much like the previous pages already express.
To your final point, how anyone on this forum treats their pet is their own business but I would hope that their actions/treatments are driven behind solid, rational thinking and experiences (both your own and from others). To most people attempting to convert non-humans to vegan diets is irrational and pointless; to declare this as as a "warped political belief" is ridiculous. It's not political to point out the obvious; it's unethical and unrealistic long term to forcfully feed your pets diets their bodies are not designed to withdraw nutrients from.