I’ve had this fish in my aquarium for months now and it’s as fat and happy as can be, yet I’ve never actually seen it consume any of the food’s I’ve offered. How on earth is it getting enough to eat?”
Caribbean Chris and I often field queries like this here at Saltwater Smarts, and they pop up with some regularity on internet forums as well. So what’s the answer? How can a fish survive for a long period in a closed aquarium if it never accepts any of the foods it’s offered? Well, there are a few potential explanations as well as a worrisome possibility to consider.
It’s feasting on resident microfauna and/or flora
The fish could, for example, be feasting on amphipods, copepods, worms, and other tiny invertebrate “bugs” that inhabit live rock and live sand. Your system could be crawling with these critters without you even being aware of it unless you check out your system with a flashlight after dark.
If the specimen in question is herbivorous or omnivorous and your tank has a lush, self-replenishing crop of algae growing on the rockwork and other hard surfaces, it may be getting enough to eat simply by grazing the algae and/or consuming the detritus and tiny organisms trapped in it. Of course, if your tank is relatively devoid of algae, you can rule out this possibility.
It’s nibbling leftovers in the rockwork
Even in systems with fairly vigorous water movement, some food bits drifting through the rockwork are bound to get trapped. A smaller fish might find enough to sustain itself in this manner, especially if there are ample microfauna in the tank to supplement the leftover bits.
The sixline wrasse (Pseudocheilinus hexataenia) that has resided in my 125-gallon tank for upwards of nine years now gets much of its food in this manner. It tends to be intimidated by larger tankmates at feeding times, so it hides in the rockwork and just snatches the odd bits that drift really close by or into the caves and crevices it hides in. You have to watch the fish pretty closely to see that it actually gets something to eat, but it does. Of course, my tank also has lots of resident “pods” for it to nibble on.
It’s not actually getting enough to eat at all
This is the worrisome possibility I mentioned above. Many fish can go a long time without food before manifesting any obvious signs of starvation, and starving fish can cling to life for many months before finally succumbing. So, a fish that appears “fat and happy” despite not apparently eating anything may actually be in real trouble.
Also, keep in mind that some fish species, such as mandarins and other dragonets, depend on live pods or similar microfauna for their survival and rarely accept nonliving fare. The trouble is, these types of fish can easily exhaust the resident pod population with their constant hunting and pecking and then starve to death. This depletion can occur over the course of many months, so you can’t assume they’ll continue to sustain themselves in the long term unless you somehow replenish the pods or regularly offer similar live food items.