suggestions?

ok, so at the moment, I have a relatively open 20g with two tiger barbs, two albino rosy barbs, a ropefish, a rainbow shark and a common pleco.
I'm looking to increase the population/quality of fish in the tank, any suggestions?

I personally like neon tetras. Some corey cats could be nice too...?

This is a nice planted trick I want to try when I get a bigger tank. It's a bit of a chore, but would look really nice in there

Moss Wall

Before doing neons I'd go for cardinal tetras (larger than neons with the bottom half red).

Loaches are pretty sweet fish, I'd go with a red tail botia or a tiger loach. They are super active fish.

And Maybe a flame dwarf gourami or some Pseudomugil Rainbows.

Just be sure to keep up with water changes since this will be a heavier bio load for a 20.

+1 on the all of the above. You could also do otocinclus.

Great ideas thanks guys.
I might try that moss wall trick and get a few loaches.
The reason I wouldn't try smaller fish like otos, cardinals, and neons is my rainbow shark. He likes to chase and the fish needs to be of comparable size and speed.
Pseudomugil Rainbows look pretty cool, I'd never heard of them before, might look more into those as well.
as far as plants go, I also am planning on adding more to the tank to curb stress caused by the rainbow on my other fish.
any other suggestions for plants?

I never really did live plants, but I'm know a few others on here have plenty. As far as the loaches go, they are very active fish and may chase down other fish so they should be good with a rainbow shark.

Here's what I've tried in my tank and have liked alot:

Hygrophyla: This one grows like crazy, but has been super pretty. I've been calling it the parsley plant because it resembles the herb.

Ludwigia: This one is so pretty, and does extremely well in the background with plenty of light. Mine has climbed out of the tank though, so that may not be something you like.

Aurea: This one is pretty tolerant, but looks great bunched up. Again without a enough lighting it will climb out of your tank, but it still looks nice, you just need to trim it. In my tank it's kinda gotten smothered by the parsley plant and ludwigia, I will try to save it before it dies.

Dwarf Sagitterius: Awesome foreground plant, that's been doing wonderful in my tiny tank. Grown a new head since I got it.

Others: Anubias, Moss Balls and Java Moss (though I'd suggest a finer moss for your wall (maybe christmas moss or willow moss) I've had these since the beginning and all are doing fine . Take a look at my thread for my pico if you want to see how they look. It will of course be different in a 20g. Oh and I'd highly advise against any cabomba species.