What Gives with that Marine Fish that Never Eats?

Sometimes you have a fish in your aquarium you never see eating, but is that a problem? It depends.

Sometimes you have a fish in your aquarium you never see eating, but is that a problem? It depends.


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.

Photo credit: Eric Strand

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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.

Comments

  1. If the fish is fat and healthy and most importantly, living, it is probably getting enough to eat. I once had a Brutlyd or cusk eel which looks like a bright yellow eel for 18 years and I never saw it eat. I actually never even saw it unless I looked at night with a flashlight. I killed it by accident but even then, I doubt it was hungry.

  2. I am very new to this hobby. My tank is a 36gal trickle system and has been up since August. It has gone through all of the cycles and the LFS gave me the okay about 3 weeks ago to but in fish. I have two PJ’s in my tank. Live Rocks and Live Sand. I have snails and hermit crabs. I just added two bleny’s. I have one small Green Star and PomPom Corals. What I have noticed when I deal with the LFS is they are very happy to tell you in the beginning how much they will help you but every time I visit the store now that I am set up they have very little time for my questions. When you know as little as I do you it is very stressful to not have help. The one thing I need to upgrade in my tank is the lighting and I need help in what to buy.

    • Jeff Kurtz says:

      Hi Deb! It is extremely frustrating when you can’t get staffers at your LFS to give your questions the time they deserve. I’m not sure whether you’ve seen it yet or not, but here’s a link to a post we ran some time ago on reef lighting that will provide a basic overview: http://www.saltwatersmarts.com/intro-to-reef-aquarium-lighting-1405/. Feel free to give it a look and let us know if you have further questions we can help with.

      • Hey my 36gal tank is in a room of my house that’s an addition. There’s is air and heat in the room but we have to switch between the two heat or air the temp in
        The room can fluctuate. My tank has a fluval heater for the size for the size of my tank. Since the cool nights and warm days my temp has also fluctuated between 73 and 77. Any suggestions on how I can help the heater keep up. Moving the tank is not an option. Anyone ever use two heaters!
        Of my tank.

        • Jeff Kurtz says:

          People with larger aquariums commonly use two heaters, with each providing half the necessary wattage. Using the 3- to 5-watt-per-gallon rule, your tank would need somewhere between 110 and 180 watts of heating power. Rather than use one heater rated for that wattage, you could go with two heaters, each rated for half that wattage–somewhere between 55 and 90 watts (or as close to those wattages as you can actually find on the market). Using two heaters helps distribute the heat throughout the tank better, and if one fails, you still have another in operation to prevent a major temperature plunge until you can replace the defective one.

          • Thank you. I will look into this. I think good through the winter I will need to distribute the heat. Thank you again. I am having so much trouble with the different forums. Made some made because I high jacked a thread. Didn’t even know what I did. Lol

  3. I have a huge emperor angel (15-16 inches). Can’t get to eat. Tried every thing from clams on 1/2 shell, cockles, squid, garlic soak. You name it. Today I’m going to try sponge. Always swimming normal and seems to be looking but never even picking. Does have some fungus on spikes protruding back from gills. Think that could be cause? Any suggestions? Thanx Joel

    • Jeff Kurtz says:

      Hi Joel! Adult emperor angels can be very reluctant feeders in captivity (hard to teach the proverbial old dog new tricks). Have you had the specimen long? Also, can you provide any additional information–tank size, water parameters, tankmates, etc.?

      I’m not sure what to make of the material protruding back from the gills, though it could be excess mucus produced for any number of reasons. Is the specimen exhibiting any other unusual behaviors?

      • Joel klein says:

        My emperor is about 16 inches. Have about a month . No interaction among tank mates. Tank size 220 gal.
        Specific gravity 1020. Temp. 78 deg..I’m familiar with large angels can be tough to start feeding. Hoping may have suggestion of something I did not try. Thank you for your response.

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