Fish, plants, and substrate.

[aircap1]

Hello!
I've been thinking about getting a community fish tank, and I don't know what other fish I should get that will go along well and how many I should get.
Tank size is 40-50 (We have extra fish tanks in storage but I don't know what the dimensions are.)
pH 75~ is my goal
Hardness- up to about 12 max
I'm a beginner and have almost 0 ideas on what I should do but fish I'm looking to get are,
Wrestling Halfbeaks,
Black Widow Tetras,
and Panda Corydoras
Thinking about adding hatchetfish.

I already have a decent light fixture and should have a really good filter.
Any chance I can make a 75 gallon fish tank filter work for a 40-55 gallon?

For plants I'm looking for some tall plants and some short ones.

Theme is just dark black, to show off the silver of the Halfbeaks, so fish that are silver are preferred.

And as far as substrate goes, I have no idea.

Thank you for your help!

Quick edit: There is a 55 gallon tall tank in storage that I remember.

 

[SaraB]

Ok both the halfbeaks and hatchet fish will occupy the top of the tank so you want something to go in the middle. If you want a really nice silver then I reccomend diamond tetras.
PH isn't too important if you buy your fish locally.
Anubias is one of my favorite plants. I have an 11 year old anubias that I managed to get 6 other plants from propagating it. It stays short.
Good taller plants include: Amazon sword, java fern, water wisteria can get a decent height, if you want red scarlet temple can get up to 12".
If you are getting corydoras then you should stay away from sharper substrates. Dirt is dark if you wanted to try it. There is also a blasting sand (can't think of the name,) that is black. If substrate colour dosen't matter then I highly recommend pool filter sand. It light but it's cheap and stays compact (no sand clouds).
Good luck and have fun.