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Anyone have information on freshwater flounder ?

Doc Aquatic

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@Doc Aquatic; there is only one "Flatfish" Achirus Errans that can spend its' whole life in the home aquarium, all others live in salt or brackish water and swim into freshwater to spawn and do not do well in the home aquarium. Yours' looks like a fine specimen.

They are blackwater fish that prefer warm ( 80 to 85 degrees), soft, and acidic water. They only eat live food so it's best to keep some feeder Guppies in your tank so they'll have babies for the Brazilian Freshwater Sole to eat.  

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That actually appears to be a tonguefish. I believe there are roughly 10 species in the aquarium trade masquerading as freshwater flounder.


Tonguefish and flatfish are extremely hard to classify without the specimen in hand because you need to look at the accessory dorsal branch as well as gill raker count. Going off coloration alone is incredibly inaccurate. I want to put it in the Achirus genus though based off what I can gather of vague mouth details and eye distance. 


Species doesn't really matter here because all of them are indeed brackish fish that are found in estuaries. This means salinity doesn't necessarily need to be consistent or high. I've heard of people keeping them successfully at as low as 1.005 with bumblee gobies. Granted gobies are liable to be prey for this ambush predator. However, tonguefish are lazy and prefer easier meals of worms and other inverts they can find in the sand. They have quite an adept nose, but poor eyesight.


Sand/mud is required substrate as these are very fragile fish and will cut themselves on any abrasives.


I've heard of the true freshwater variety @Gator is referring to showing up in rare instances. However they are distinct in that they have cirri around their mouth. Those could be cirri at the tip of the mouth, but to my eyes I think it's the mouth.

For reference "cirri" are the tufts on top of this blennies head that make it look like he just woke up.


Photo credit: Florida Museum

Edited by Biotope Biologist
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I caught one that looked alot like that about 10 years ago. I put it in a freshwater sand bottom community tank of natives and wild caught fish. It went straight to the bottom and burrowed in the sand. I only saw its eyes in the sand one other time about a week later. Not sure if it eventually died or was always hiding. When we took the tank down we didn't find it. I never caught another. It was collected from freshwater but there are storm drains that can introduce saltwater to the lakes/ponds.

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