Carnivorous fish eat meat, herbivorous fish eat algae/plants, and omnivorous fish eat both. That’s the order of things, and any fish we buy for our aquariums should fit nicely into one of these categories so we know exactly what to feed it, right?
Well, if that’s the case, why on earth do my blue-chin triggers (Xanthichthys auromarginatus)—carnivores by nature—always beat my tangs and foxface to the dried algae sheets I offer and actually eat the lion’s share? After all, FishBase describes X. auromarginatus as “[forming] loose aggregations a few meters above the bottom where it feeds on zooplankton, particularly copepods.” Nowhere in this statement do you see, “Oh yeah, and it likes to tear into algae from time to time, too!”
On the flipside of the coin, just as my triggers seem to enjoy ordering “off-menu,” my herbivorous yellow tang (Zebrasoma flavescens), Atlantic blue tang (Acanthurus coeruleus), and one-spot foxface (Siganus unimaculatus) will greedily gobble up any meaty items that they can fit in their mouths as well.
There are a few points to consider about this type of behavior in aquarium fish, which, by the way, is not at all unusual:
First of all, most fishes’ diets aren’t as exclusive one way or the other as we’d like to believe. For example, we tend to think of the aforementioned foxface as a dedicated herbivore, but its natural menu also includes a fair amount of zooplankton. You also have to consider that even the most dedicated predator ingests the plant material contained within the gut of its prey items and the most dedicated herbivore incidentally consumes the miniscule/microscopic fauna clinging to the algae it consumes.
Second, many fish are opportunistic feeders to at least some extent, meaning they’ll eat what they can when they can, especially if they’re hungry enough and their preferred food is not available. This is actually a good thing because most of the fare we offer our aquarium fish bears only passing (if any) resemblance to the stuff they eat in nature. If most of the fish we keep weren’t opportunistic, many more of them would starve to death in our systems.
So why does any of this matter? For me, it’s a reminder that I can’t get lackadaisical about feeding just because my fish make it easy for me to do so. My triggers may be happy to rip into sheets of nori or red marine algae, but a steady diet of that and little else will compromise their health. Ditto for herbivores fed nothing but mysids. Aquarium fish can eat only what we offer them, so it’s always incumbent upon us to make sure they’re getting the right types of food in proper proportion.