I agree that a showcase fish should be what is really wanted by the tank owner.
For me a centerpiece fish is either Panda Garra or ropes. Most would not see these fish the way I do as I'm more into behavior than flashy appearance. As to ropes they are just cool and have a LOT of personality. Pandas are also cool but can be a bit of a pain when doing maintenance in the tank as they will latch onto your hand and go for a free ride. As to size Pandas get to 3-3.5 inches and ropes get to 8 inches or longer. Actually ropes can get much bigger but I have never experienced one getting over 8 inches.
I'm not really recommending either fish to you as they are my favorites, not yours. First off ropes would not like your tank as it is fast moving and they like still to slow moving water.
Now Pandas are a different thing. If you like fish that will interact with you they will climb all over your hand. They like faster water and are one of the best tank cleaners around, even eating black beard algae. Still not saying Pandas are what you should get but, if you are not sure, may be worth a look. They are not fancy beautiful but I DO find them attractive. One thing that is essential is to keep your nitrates low as 20PPM is pretty much fatal.
Looks sort of like a loach and is often mistaken for such. Actually they are more in-line with carp and minnows. Oh, and they like bubbles to play with. Here is what they look like.
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