Identification Help From Flatfish,flounder,sole Pros.

[Sandalphon]

I just received two 4-5" flatfish. They were sold to me under the the Achirus Lineatus name (Lined sole, brackish/marine). However, despite inventory check that said they had the two flatfish in stock, the seller said it was out of stock. After a few phone calls I got in touch with a kid in the department that found them in the 100% freshwater area.
I currently have them in their own brackish tank since I figured that was the safest bet for now.

My big worry is, the seller also stocks Catathyridium jenynsii, a Freshwater south American sole. I can barely tell the difference between these two species (Catathyridium jenynsii & Achirus Lineatus), and want to make sure I have the right water conditions. Both my new sole have lots of small blacks circular spots on the tail and fins yet no spots on the body, and very thin black lines on the body.

Here are the photos from the shop below.
Achirus Lineatus:

Catathyridium jenynsii:

I'm no fish photographer, here are the best pics I could take. I used a turkey baster and blew all the sand off of each of them to expose details. I tried using a flashlight and no flash on the cam, but it all comes out as rubbish.

and flashlight in the facial region:

Of course, they could just be common Hogchokers (Trinectes maculatus)?


The bottom side of these fellas is mostly white with big dark spots around the tail/anal-fin area.
any help from you pros would be GREATLY appreciated.

 

[nmonks]

The big problem here is that soles/flounders can change colour. But there are photos of T. maculatus from above and below at Fishbase. And simply because we use this name and a couple of others in the hobby, it DOES NOT mean that other, similar-looking species don't get imported. Outside of an ichthyology department, positively identifying flatfish is extremely hard.

For what it's worth, I'd keep these in low-end brackish, say, 1.003, until they're feeding. Once settled, you could lower the salinity, and if they carry on feeding happily, acclimate them to plain freshwater. If they stop feeding or start showing unusual behaviour or poor health, up the salinity. Keeping freshwater soles in slightly brackish water shouldn't cause them any harm at all.

Cheers, Neale