Fish With Eyes On One Side Of Head – A Complete Guide To Keeping Flatf
Have you ever looked into a tank and felt like the substrate was looking back at you? Finding a fish with eyes on one side of head is one of the most surreal experiences a hobbyist can have.
I remember the first time I saw a freshwater sole blending perfectly into the sand. It was so well-camouflaged that I almost missed it, and when it finally moved, I was hooked on these bizarre, pancake-shaped wonders.
If you are looking to add a truly unique specimen to your collection, you are in the right place. In this guide, I’ll show you exactly how to care for these masters of disguise so they can thrive in your home aquarium.
Why Do They Look Like That? The Science of Fish With Eyes On One Side Of Head
The fish with eyes on one side of head belongs to the order Pleuronectiformes, commonly known as flatfish. This group includes well-known sea creatures like flounders, soles, halibuts, and turbots.
What makes them so special is their incredible metamorphosis. They actually start their lives looking like “normal” fish, with one eye on each side of their head, swimming vertically in the water column.
As they grow, a miraculous biological shift occurs. One eye literally migrates across the top of the skull to join the other on the “up” side, and the fish begins swimming horizontally along the bottom.
The Advantage of Asymmetry
This isn’t just a weird evolutionary quirk; it is a highly specialized survival strategy. By having both eyes on the top side, these fish can remain completely buried in the sand while keeping a 360-degree watch for prey.
Being flat allows them to exert very little energy while waiting for a meal to swim by. It also makes them nearly invisible to predators looking down from above, especially since they can change their skin color to match the floor.
Top Flatfish Species for the Home Aquarium
While most flatfish are massive saltwater giants, there are several species perfectly suited for smaller home setups. However, you need to be careful, as many are often mislabeled in pet stores.
Most “freshwater” flounders sold in the hobby are actually estuarine fish. This means they might need a bit of salt (brackish water) to live a long, healthy life, though some true freshwater species do exist.
The Hogchoker (Trinectes maculatus)
The Hogchoker is perhaps the most common fish with eyes on one side of head found in local fish stores. They are hardy, stay relatively small (around 3-6 inches), and have a beautiful mottled brown pattern.
While they can survive in freshwater for a while, I have found that they truly thrive when kept in slightly brackish conditions. If you’re a beginner, this is a great species to start with if you’re willing to learn about salinity.
The Freshwater Sole (Brachirus pan)
If you are looking for a true freshwater inhabitant, the Brachirus pan is a fantastic choice. These are often found in Southeast Asian rivers and stay quite small, making them manageable for intermediate keepers.
They have a more rounded shape compared to the Hogchoker. They are peaceful but can be quite shy, so don’t expect them to be the “star” of the tank during the middle of the day.
The South American Freshwater Flounder
Often coming from the Amazon basin, these are rare but highly sought after. They are strictly freshwater and appreciate softer, slightly acidic water, which is a bit different from their brackish cousins.
Setting Up the Perfect Benthic Habitat
Because a fish with eyes on one side of head spends 99% of its life on the bottom, the “footprint” of your tank is much more important than the height. You want a long, wide tank rather than a tall one.
For a single specimen, a 20-gallon long tank is usually the minimum I recommend. This gives them enough “runway” to glide along the bottom and enough surface area for oxygen exchange.
The Importance of Substrate
This is the most critical part of your setup. You must use a soft, fine sand. Avoid gravel or coarse aragonite at all costs, as these sharp edges can scratch their delicate underbellies.
These fish love to “shimmy” into the sand until only their eyes are visible. If the substrate is too heavy or sharp, they won’t be able to bury themselves, which leads to high stress and a weakened immune system.
Filtration and Flow
Flatfish aren’t the strongest swimmers, so you don’t want a “washing machine” effect in the tank. A gentle flow is best. However, because they eat high-protein meaty foods, you need strong biological filtration.
I highly recommend using a high-quality sponge filter or a canister filter with a spray bar to distribute the flow. This keeps the water clean without blowing your flatfish across the tank.
Dietary Needs and Feeding Strategies
Feeding a fish with eyes on one side of head can be a bit of a challenge at first. They are natural-born predators, and many specimens arriving at pet stores are wild-caught, meaning they might not recognize flakes or pellets as food.
In the wild, they eat small crustaceans, worms, and tiny fish. To get them settled in, you should start with live foods to trigger their hunting instinct.
Best Foods for Flatfish
- Live Blackworms: These are the “gold standard” for getting a picky flounder to eat.
- Frozen Bloodworms: Once they are settled, most will readily accept frozen bloodworms dropped near their face.
- Frozen Mysis Shrimp: Great for nutrition, though you might need to use feeding tongs to “wiggle” the shrimp to get their attention.
- Chopped Earthworms: A fantastic high-protein snack for larger specimens.
The “Feeding Station” Technique
Because flatfish are slow and camouflage themselves, faster tank mates will often steal their food before they even realize it’s there. I’ve found that using a long pair of aquarium tongs is the best way to ensure they eat.
Simply hold the food a few inches in front of their eyes. When they sense the movement, they will “pounce” with a surprising burst of speed. It’s one of the coolest things to watch in the hobby!
Compatible Tank Mates for Flatfish
Choosing neighbors for your flatfish requires a bit of strategy. You want fish that occupy the upper levels of the tank and aren’t aggressive enough to nip at the flounder’s delicate fins.
Since the fish with eyes on one side of head is a bottom-dweller, avoid other territorial bottom fish like large Cichlids or aggressive Catfish. You don’t want a “turf war” happening on the sand.
Great Tank Mates
- Glass Catfish: They stay in the middle water column and are very peaceful.
- Peaceful Tetras: Larger tetras like Rummy Nose or Diamonds work well.
- Livebearers (for brackish): Mollies and Guppies are excellent if you are keeping your flatfish in a brackish setup.
- Bamboo Shrimp: Usually large enough not to be eaten, and they stay off the bottom.
Fish to Avoid
Avoid small “nano” fish or tiny shrimp like Cherry Shrimp. If it can fit in the flounder’s mouth, it will eventually become an expensive midnight snack. Also, avoid “nippy” fish like Tiger Barbs that might peck at the flounder’s eyes.
Maintaining Water Quality and Health
Flatfish are quite sensitive to water parameters, especially nitrates. Since they are in constant contact with the substrate, any buildup of waste or “mulm” can cause skin infections or “red belly” disease.
I recommend a 25-30% water change every single week. Use a gravel vacuum to gently sip debris off the surface of the sand, but try not to disturb the deeper layers too much where your fish might be hiding.
Acclimation is Key
When you first bring your fish with eyes on one side of head home, do not just “plop and drop.” They are sensitive to changes in pH and salinity. Use the drip acclimation method over the course of an hour.
This slow transition helps their internal systems adjust to your water chemistry, significantly reducing the risk of shock. Remember, a stressed fish is a fish that won’t eat!
Frequently Asked Questions
Are all flatfish saltwater fish?
While the majority are marine, there are several species that live in brackish or pure freshwater. Always research the specific scientific name of the fish you are buying to ensure you have the right salt levels.
Why is my flounder turning white?
Flatfish are masters of camouflage. If they are on light sand, they will turn pale. However, if the fish looks “milky” or has fuzzy patches, it could be a fungal infection or a sign of extreme stress.
How big do aquarium flounders get?
Most species kept in the hobby, like the Hogchoker, stay between 3 and 8 inches. However, always double-check, as some marine species that look similar can grow to several feet!
Can I keep more than one in a tank?
Yes, as long as the tank is large enough. They aren’t particularly social, but they aren’t usually aggressive toward their own kind unless food is very scarce. Ensure there is enough floor space for everyone to have a “spot.”
Do they need a heater?
Most tropical flatfish prefer temperatures between 74°F and 82°F (23°C – 28°C). A stable temperature is vital for their metabolism and immune system.
Conclusion
Keeping a fish with eyes on one side of head is one of the most rewarding challenges an aquarist can take on. They offer a window into one of nature’s most bizarre evolutionary paths and provide a totally different dynamic to your tank.
By providing a soft sandy substrate, ensuring they get enough meaty foods, and maintaining pristine water quality, you can enjoy these “living shadows” for many years.
Don’t be discouraged if they seem shy at first! Once they realize you are the “bringer of worms,” they often become quite responsive and will even wait at the front of the glass for their meals. Happy fish keeping!
