Hi !
I come back with some news.
So, i added more hiding places, more flow and more "oxygen" ( i added a water pump midwater, this creates a kind of fountain effect). They now are swimming like crazy everywhere in the tank, i am so happy to see them behaving like that !

Thanks a lot for these answers !
@MissNoodle : There is tannins in the aquarium but i can add more yes. The tank is heavily planted (mostly with epiphytes on wood) with lots of Driftwood + floating plants, i would say that 60% of the tank is really dark.
@Shane : I had a noisy skimmer i changed 2 ...

Hello everyone,

I am new here ^^ and French, so, I apologize for the mistakes i will write :)
So it's been days and days I am looking for an answer on the web but i can't find any solution.
2 months ago I added 20 pygmy corys to my 20gal long tank. There are just some shrimps with them. Ph is 6 ...