Longhorn Cowfish Care Guide

Scientific NameLactoria cornuta
Final Tank Size250 Gallons
Water Parameters72-78 F, pH 8-8.4, Salinity 1.020-1.025
The Longhorn Cowfish is a popular boxfish that people love to refer as swimming cheese or the yellow box fish, not to be confused with the actual yellow boxfish. Despite their cute appearance the Longhorn Cowfish is one of the riskiest to own fish and can cause thousand dollar tanks to die in the blink of an eye. This fish is not to be kept by anyone who cannot afford to lose their tank. They are for experts only.

What makes this fish such a big deal? when threatened or stressed out they will release a neurotoxin, known as ostracitoxin or Pahutoxin. This toxin will quickly kill everything in the tank, sometimes even the boxfish who released the toxins as well. In the wild the toxins would usually disperse and not be too powerful but a closed loop of an aquarium makes their toxins incredibly potent. If this toxin is released it is essentially impossible to save anything in the aquarium. The aquarium itself would need to be cleaned to be used again and all rocks, sand and coral should be discarded. Trying to re use poisoned rocks could kill anything added to the newly made tank. I personally would not try to save the aquarium and would buy an entirely new tank rather than risk keeping some toxins or having cleaning solution stay in the aquarium.

Wild Longhorn Cowfish will often reach the full 20 inch size. In the aquarium they usually stay under 16 inches even when fed the best diets possible. Their bodies are hard scaled and cannot bend. It is easy to identify them using their two large horns and two long spines under their tail fin. Their eyes can move independently like a chameleon, helping them keep track of food and their tankmates. They lack pelvic fins and are very slow swimmers. These two traits make them incredibly easy to catch by hand. When caught they will start grunting. Do not catch your Longhorn Cowfish as it is likely to release toxins if you do.

There is no difference between male and female cowfish.

Is the Longhorn Cowfish Reef Safe?

The cowfish is some what reef safe, mostly just coral safe. They will not eat corals and are fine with shrimp while they are not big. The larger they get the more danger inverts will be in. This means they can be kept with most shrimp for a few years before they start hunting the large decorative shrimp we keep in the aquarium. Cleaners, coral banded and peppermint shrimp all do well in the early years with the Longhorn Cowfish.

Worms will not be safe with the Longhorn Cowfish even if they burrow under the sand. The cowfish shoots jets of water from its mouth to find hiding worms and is an expert worm hunter. Likewise they will nip at mollusks, clams, sponges and scallops. Large shelled crabs should be safe from the Longhorn Cowfish even when the cowfish is large. New crabs may be harassed if they are not large and the cowfish is already well established.

The Longhorn Cowfish likes to form a relationship with acropora coral, which is a short branching SPS coral. They will host these similarly to clownfish but will not spend as much time around the coral.

Longhorn Cowfish Diet

The Longhorn Cowfish is easy enough to feed but requires more nutritious foods than most other fish. While they will accept flake foods they do not do well when fed an exclusively flake diet. Sinking pellets are difficult for them to eat as they are slow swimmers and will not usually catch the pellets before they reach the floor.

A good diet for the Longhorn Cowfish should contain:

  • Dried brine or mysis shrimp
  • Live gut loaded brine shrimp
  • Chopped table shrimp
  • Frozen foods
  • Chopped Squid
  • Clam
  • Mussels
  • Sponge based foods or sponge
  • Seaweed
  • Algae

The Longhorn Cowfish does well when fed slowly rather than large broadcast feedings. They lack pelvic fins and swim very slowly. They will usually sit in front of their food, taking small bites occasionally. This makes them harder to feed around other large fish and can leave them starving if not target fed. They will eat things like algae clipped to the aquarium glass or live rocks. Likewise they will scrape algae off of rocks, however this should not be considered a large part of their diet as algae will not grow nearly as quickly as the Longhorn Cowfish needs. Like puffers and other boxfish the cowfish will spit water out of the tank if they want attention or food.
When feeding them seaweed it is important to do two things. Store the seaweed in a dry area. This will keep the seaweed for longer and make it more durable when it enters the tank. As it re moisturizes the seaweed will begin to break down and will break apart in the aquarium much quicker. This will make it difficult to feed the Longhorn Cowfish and will also contaminate the water much quicker. The second thing to keep in mind when feeding seaweed is to remove if after an hour of adding it to the tank. If there is any remaining it should always be removed even if it is still clipped to the wall or rocks. After an hour it should be breaking down and will start to contaminate the tank.

Picky Longhorn Cowfish that are not eating should be fed live feeder shrimp like baby brine shrimp. They may have trouble catching the shrimp and may decide to eat easier foods that we normally feed the tanks. Most other fish in the tank will be quick to eat these feeder shrimp so long term use of feeder shrimp is not really possible unless you breed them in your own side culture tank. They also enjoy feeding by hand, which ensures they get the food rather than it blowing around the tank.

Longhorn Cowfish Tank Requirements

The Longhorn Cowfish is not at all picky with its aquarium. They do well in just about any aquarium that accommodates their size. The ideal tank should have plenty live rock with growing algae on most of the surfaces. While they do not need a sand bed they do enjoy hunting for worms by blowing the sand around with jets of water. Even if there are no worm sin the tank sand should still be included.

When arranging the tank it is important to remember the Longhorn Cowfish has a large, hard body. They cannot squeeze between rocks and small caverns like a lot of other large fish do. Try to have a few caverns and paths that they can easily swim through without touching the sides. Overhangs are also good to have and can provide large flat surfaces for the Longhorn Cowfish to graze on algae.

They live near river mouths while young but move towards heavily planted areas with corals and rocks. They appreciate a decent amount of flow but should not be placed in a high flow only tank. Try to keep the tank more of a low flow aquarium to ensure they don’t get stressed out. Do not assume they will be able to claim the low flow areas in the tank, as other aggressive fish or large eels may claim these areas. This could leave to a stressed Longhorn Cowfish that releases toxins and kills everything in the tank.

Any aquarium that houses this fish should be built around them. Trying to keep the Longhorn Cowfish in a tank not meant for it can easily result in a toxin bombed tank. Be extremely proactive with any issues you see happening with the cowfish. Do not put these issues off for even an hour.

Longhorn Cowfish Tankmates

The Longhorn Cowfish is peaceful and should not start a fight with anything but other boxfish and sometimes similar looking puffer fish. New small fish may be chased occasionally but should not be in danger when around the cowfish. They can sometimes be the target of bullying but should generally be ignored due to their hard bodies and toxic defenses.

Large aggressive fish should be avoided entirely. While they may get along with the Longhorn Cowfish in most situations it only takes a few minutes of chasing for the cowfish to release its toxins and wipe out the entire tank. Keeping large fish like triggers or groupers is always putting the tank in extreme danger. Only do this if you do not mind losing the 200+ gallon aquarium and everything inside it overnight. Large fish like the Scorpionfish can eat the Longhorn Cowfish whole when they are small. However they will quickly be spit out due to their hard bodies and toxins.

Good tankmates are:

  • Dwarf Angelfish
  • Clownfish
  • Cardinalfish
  • Chromis
  • Butterfly Fish
  • Peaceful Angelfish
  • Wrasse
  • Anthias

Do not keep two boxfish together ever. They will frequently fight one another even in large tanks. This will usually lead to them releasing their toxins. Boxfish are not immune to an identical boxfishes poison so both will usually die when kept together. The best case is only one dies. Once dead the Longhorn Cowfish cannot create or release the toxins.

Most shelled inverts will be fine when kept with the Longhorn Cowfish. Snails may be eaten if they get flipped over. Any shrimp that are over two inches should be fine but more aggressive Longhorn Cowfish will still eat them. All crabs will be fine with them but my have a few test nips taken at them.

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