Sorry to side step a little but have you looked at Central American eartheaters like thorichthys?
They would thrive in your water,they would do great in a decent sized group and they average 6 to 7 inches for males and about a third of the size for females.
I'm a big fan of these fish.They are colourful, entertaining will breed in a community setting without too much aggression.
Here a some thorichthys helleri I kept. These pictures are of a 4 foot tank.
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And here are a pair in breeding colours
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