Ember Tetras

According to Seriously Fish, ember tetras need 5.0 - 7.0 and GH 18 - 179ppm.
179ppm = 10 German deg so I'm afraid your GH of 21+ is too high for them.


With water that hard, I am struggling to think of fish that would work in a small tank. The obvious hard water fish, Rift Lake cichlids, unfortunately need big tanks.

There are a couple of livebearers that would work but they are not common.
Heterandria formosa - very small fish, pH up to 8, hardness up to 20 german deg.
Neohetrandria elegans - also very small, pH up to 8, GH up to 25.

You might be able to find a shop that would order some in. And if you had mixed sexes, you might be able to sell the fry on with them being so rare.

A shrimp tank would also be an idea, provided you keep to the common red cherry shrimp or relatives (he more unusual types almost always need soft water and are ridiculously expensive). Shrimps are more entertaining than many fish, especially if they are on their own or with very small fish so that they feel relaxed and can exhibit natural behaviour.

Shrimp are pretty robust, mine only get fish food the same as everybody else. They are good scavengers and will eat most things. They can be pretty small when you get them but apparently young shrimp adapt better to new conditions than older ones, or so they say. It's never a good idea to stock a tank with shrimps or fish shortly before you go away, always sensible to stock up when you know you will be around for a few weeks to keep an eye on things