It Can Be Difficult to Tell If a Fire Eel is Actually Eating

Q. I have a 55-gallon aquarium with black sand and a large castle, which is home to a fire eel, four feeder fish and a few ghost shrimp. My fire eel will never come out of the castle. It won’t even come out to eat. I tried feeding it krill, bloodworms, flakes, feeder fish and shrimp, but it doesn’t seem to eat any of it. I don’t think it’s scared because I’ve had it for about two months now. Is this normal, or should I be concerned?
-- Scott Henning
Herron, Mississippi

A. I’d be concerned. A fish that’s not eating is usually a fish that’s not happy or healthy. There are exceptions, such as species that refuse to eat while protecting fry or eggs; and I have seen instances where large piranhas or catfish have gone a couple of months before they’d feed in the aquarium. The reasons were unknown, but it’s possible that these wild fish were caught on a hook and had mouth injuries that kept them from feeding initially. So it may not be too late for your fish, even though it apparently has gone so long without food. I can’t say that I’ve ever seen a fire eel (Mastacembelus erythrotaenia) that wouldn’t eat, but they are sometimes caught on hooks (they are a food fish in Southeast Asia).

It’s not unusual for fire eels to hide all day. They are nocturnal and normally only feed at night. So, it’s not always easy to tell if they are feeding. Most quickly learn to come out at feeding time, however. You didn’t say how large your eel is, but the first thing I’d check is to make sure it can easily come out of the castle. I’ve seen several instances of this fish finding its way into a ceramic ornament and then becoming trapped as it squeezed halfway out one of the holes, only to discover it was too big to continue. The ornament had to be broken to free the fish. If the ornament has several holes, perhaps the fish just hasn’t figured out which are large enough to exit.

Next, check water quality. Nothing affects a fish’s appetite more than poor water quality. Low pH, or high ammonia or nitrites can cause a fish to stop eating. Try making some extra water changes to stimulate feeding.

It looks like you are offering a proper diet, except for the flakes — fire eels won’t eat them. So, flakes will just end up polluting the tank. Feeding flakes to the feeder fish and ghost shrimp is fine, however, and is a great way to pack extra nutrition into your fire eel when it starts feeding. The bloodworms may also be too small for the eel. These fish are sold in sizes from 4 to 24 inches. The little ones will happily eat frozen bloodworms, but the large specimens may not mess with such small food. You also didn’t mention which kind of feeder fish you offered. Again, choosing a feeder fish of the proper size could make a difference. There’s quite a size range between feeder guppies, rosy red minnows and feeder goldfish, for example. Live blackworms are relished. Another tempting food to try is earthworms. They’ll stay alive until eaten, though some end up hiding in the gravel.

Finally, you might merely need to add some more feeder fish or ghost shrimp to get things going. If there are only a few, the eel may have a harder time catching one. Another weird thing that sometimes happens when offering feeder fish is that the predator becomes accustomed to them. There are countless reports of a single feeder goldfish — the last one in the batch — that never gets eaten until a new batch is thrown into the tank. It makes hobbyists think that predator and prey became friends. More likely, it’s just too hard to catch that last one, or the predator gets so used to seeing the fish swim around that it starts viewing it as part of the environment instead of part of the menu.

Driftwood and Tannins

Q. I have a 50-gallon tank and would eventually like to own discus. I have heard that you should add driftwood or coconut shells to a discus tank to make it more acidic, due to tannins. I have also heard that for fish needing alkaline water, you can boil the coconuts to remove the tannins, but would this be bad for my setup? Should I boil them a little for sanitation purposes but still let the water go “black” to be more like the natural water?
-- Forrest Phillips
Alexandria, Virginia

A. Driftwood is a wonderful decoration for freshwater aquariums. It adds natural beauty to the aquatic environment, and no two pieces are alike. Coconut shells — usually half-shells — are sometimes used by hobbyists to create spawning caves for fish or to provide cover for shy species. These wooden decorations have the ability to release substances as they break down, especially tannins, which can help soften and acidify the water. Tannins can color the water yellow to brown like tea. This darkened, soft, acidic water is often called “blackwater” because deep rivers of this type appear black.

Discus prefer soft, acid water and are found in blackwater habitats in the wild. Many hobbyists use peat, driftwood or coconut shells to help duplicate this effect. Do not boil your wooden decorations, unless you want to reduce or destroy their ability to leach tannins and color the water. The degree of leaching varies greatly. Malaysian driftwood probably leaches the most tannins, followed by African driftwood (sometimes called “mopani wood”). Both of these are self-sinking, too. The American driftwoods that sell attached to a slate base (to help them sink) leach very few tannins. I’ve never used coconut shells, though, so I don’t know where they fall on the scale. Happy fishkeeping!