Yellow Boxfish Care Guide

Scientific NameOstracion cubicus
DifficultyExpert
Minimum Tank Size125 Gallons
DietOmnivore
Water Parameters72-78 F, pH 8-8.4, Salinity 1.020-1.025
AggressionPeaceful
Size18"
The Yellow Boxfish, also known as Cubicus Boxfish, is a very popular but extremely dangerous saltwater fish. They have a unique hard body shaped like a box, eyes that can move independently of each other and a beautiful polka dot body. They are slow swimmers and become quite large. Combine this with their wide body and the Yellow Boxfish requires a lot of feeding and space.

The Yellow Boxfish is for experts only. Even then experts may lose expensive setups and long owned fish if the Yellow Boxfish is not handled properly.

What makes them so demanding? when threatened or stressed the Yellow Boxfish releases a strong neurotoxin that will kill everything in the aquarium, often even themselves or other boxfish. In the wild these toxins would disperse and drive off predators. In our enclosed aquariums this isn’t possible and instead the toxins are locked into a small area, making them incredibly potent. If you do not notice these toxins right away then everything in the tank will be lost. The toxins are most easily noticed by a foamy water surface. Tankmates will become incredibly lethargic as the toxins take effect. They can be removed from the poisoned tank and placed into a hospital tank treated with carbon to try and save them. The toxins will reach deep into rocks and filtration systems, making them essentially worthless as reusing them can kill off an aquarium unless they are expertly cleaned. In short this fish must have the ideal aquarium if you hope for them or their tankmates to survive long term.

Yellow Boxfish

Photo Credit to Derek Keats and Eugene Lim from the top adult and bottom juvenile photos respectively.

Boxfish usually see stunted growth in the aquarium, meaning their max size is more likely to be 14 inches however they can still reach 18 inches if their tank is large and not restrictive when it comes to swimming. Their lack of pelvic fins make them very slow swimmers, so care must be taken when placing power heads and selecting tankmates that may out compete them for food. As they age their bodies will change from the bright yellow to a musky brownish yellow.

Is the Yellow Boxfish Reef Safe?

The Yellow Boxfish is not considered reef safe as they will either pick at or ignore corals. This behavior cannot be determined early onin the ownership of the fish which means any coral in the tank may have to be removed within a year or end up eaten by the Yellow Boxfish. When they do nip at corals it is only LPS corals with flowing appendages. Branching corals are typically safe with this boxfish. Even a small boxfish as seen in this video can pick at fleshy membranes of softer corals.


Inverts, other then worms, are also safe with they Yellow Boxfish as the fish is far too slow to hunt any healthy decorative shrimp. The boxfish will not go for hard shelled crabs or snails. Tube worms, flat worms and feather dusters are all frequently targeted by the Yellow Boxfish and should be avoided. This does mean the boxfish can help with large worm populations however they are not expert hunters like the wrasse when it comes to rock infestations. Instead the Yellow Boxfish will look in the sand bed, blowing the sand around and catching worms this way.

This boxfish likes to swim around short branching corals, treating them similarly to how clownfish will treat anemones. They will not spend as much time around the corals as clownfish but will frequently treat the coral as their home, returning to the coral through out the day.

Yellow Boxfish Diet

The Yellow Boxfish has a fairly simple diet. They do require a large amount of food and are slow eaters. This makes feeding them around large, quick fish difficult. Avoid any aggressive eating fish as they will scare even a full sized Yellow Boxfish away from food.

The Yellow Boxfishes diet 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 biggest issue when keeping the Yellow Boxfish is getting them to eat in the first place. Most boxfish will arrive in relatively poor health and be unwilling to eat just about any food you feed them. The most enticing foods are live baby brine shrimp or ghost shrimp. Large ghost shrimp can be too quick to be caught and may need to have their tails cut to slow them down. Various worms like bloodworms also work well on newly added Yellow Boxfish. After being fed these live foods the will often start eating the easier to catch prepared foods or cut meaty food. Remember to turn off the tanks water flow so that they don’t have to chase their food around the tank.

These boxfish prefer to be fed slowly rather than large additions of food at once. They are not competitive and may need to be target fed depending on their tankmates. They will eat foods that are clipped to rocks or meaty foods like frozen clams and mussels. Likewise seaweed on the side of the tank makes for a good food source that they can take their time eating. When malnourished the Yellow Boxfish is very quick to lose its colors making it easy to know how well they are doing in the tank. Hungry boxfish will frequently swim to the top of the tank and spit water out to get the aquarists attention. This should not be encouraged as they are likely to swallow air causing buoyancy issues.
Feeding seaweed is a great way to get the Yellow Boxfish its herbivore nutrients. The two things to remember when feeding seaweed are:

  • Remove any uneaten seaweed after 1 hour
  • store the seaweed in a dry area

Seaweed is sold dried. Keeping it near the aquarium, a typically humid area, will re-hydrate the seaweed. This makes the seaweed breakdown much faster when added to the aquarium. After dry seaweed has sat in the aquarium for an hour it will start to break down no matter how dried it was when added to the tank. This will put unnecessary stress on the aquariums filtration system and increase the bio load beneficial bacteria will have to deal with.

There are typically two types of seaweed sold. Red seaweed is quicker to break down than green seaweed, however it is still good to alternate between the two types. If the Yellow Boxfish is eating both then feeding different colors will give them a mixture or nutrients.

Yellow Boxfish Tank Requirements

Yellow Boxfish are not demanding when it comes to aquarium design. As long as they have a large swimming area and a sand bed to hunt in they will be happy. Avoid creating a lot of small caverns that they cannot fit in. Their hard body makes it incredibly difficult to swim between rocks. Instead make a few large caverns and overhangs that they can take advantage of. Large flat rocks at the top of the rockwork will also give them a large amount of algae to scrape off and eat.

In the wild the Yellow Boxfish enjoys living around corals and large amounts of macro algae. Depending on their attitude towards corals you may keep them in reef tanks without them nipping or eating corals. If they do show an interest in eating corals then they should only be kept with branching corals or SPS corals they will not nip at.

The Yellow Boxfish is a slow swimming fish and should not be kept in tanks with strong currents. Avoid high flow tanks with only a few slow moving areas. Other fish will typically claim the low flow areas, keeping the boxfish in uncomfortable flow areas. High flow areas can easily blow the boxfish around, disorienting them. This increased stress can lead them to release their toxins and wipe out the tank. This is why the entire tank should be mid to low flow.

In general the aquarium should be built with the Yellow Boxfish as the focus of the tank. Adding the boxfish to a tank that was never meant for them runs a high risk of ‘bombing’ your tank with their toxins, wiping out everything in the tank. Be observant when adding the boxfish and resolve any conflicts they have with other fish quickly. They should not be allowed to be harassed while settling in the tank like many other fish. Instead they should be added before any aggressive fish.

Yellow Boxfish Tankmates

The Yellow Boxfish will generally ignore their tankmates. The only fish they will actively have issues with are similar looking puffer fish and other boxfish. They should never be kept with other boxfish as they will almost always bomb the tank with their toxins. Incredibly large tanks can attempt to get a pair of box fish but this is not advised. Males and females are incredibly difficult to differentiate. Males may be larger with darker, more brown bodies than females.

Avoid any aggressive fish when keeping the Yellow Boxfish. In the wild these fish are very safe with their toxic defenses and incredibly hard bodies. Even large scorpionfish will cannoteat the boxfish.

Despite their safety these attempts of aggression can quickly make the boxfish release its neurotoxins and wipe out the tank before the aquarists can react. Groupers and triggers are some of the more common fish you should avoid. Small, infrequent attacks like those by damselfish should not bother the hard scaled boxfish.

Good tankmates are:

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

Hard shelled inverts are good choices to keep with the Yellow Boxfish as they will often interact as the boxfish scours the sandbed where the inverts are. Shrimp will generally be fine with the boxfish and should be quick enough to escape the fish if it ever tries to catch them. Generally the Yellow Boxfish will not try to catch any of these quick inverts and will not bother anything with a hard shell. Do avoid clams, mussels and scallops as the boxfish will pick at their fleshy membranes.

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