That's not a true flounder, but a loach as you've stated. Hillstream loach seems to be the more common term in the U.S. They are a cold water fish that also require strong water flow. I've never had one, but they are interesting fish and come in some striking color/patterns.
I used to have an actual flounder when I was a kid, in my 20 gallon community tank. It lived for a while but didn't thrive and ended up dying within a year. Now I realize that it was a brackish fish, which was a factor in it's early demise. Flounders can also be difficult to feed.
I also had a fiddler crab setup, with a large sandy beach on one side and water on the other. I kept them in freshwater (mistake), and they died shortly after molting.
How long have you had your fiddlers? I'd like to try them again some day if I thought I could keep them alive. All reports I've read say that they need brackish water in order to live.