Corydoras Pygmaeus hiding

[Sobe]

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 6.8, gh 8, kh 5, temperature is 24/25 degrees Celsius. The thank is well planted, driftwood, floating plants, I have very fine sand. I change 20% of the water each week. We are just 2 persons living in my apartment.
The tank is running since 3 months now.
Everything was perfect, corys swimming a lot, everywhere, but since 2 weeks ago, I don't understand why, they just panic and hide when I or my bf move. And sadly, They now hide all the time.
I don't understand what happened. So I started to make my research and it seems that other persons have this issue, and also , suddenly. The problem is, i did not find any answer. I am thinking about different solutions to try. Add a dither fish ? Add other pygmy corys ? I am quite lost. Does anyone got the same problem and found a solution ?
Thanks for your help !
Have a nice day

[MissNoodle]

Dither fish would help calm them.

Ember tetras or a species of boraras would be an excellent choice with the shrimp too. I'd lean towards the boraras as theyre a bit more docile.

Id also make sure they have some extra cover like floating plants and maybe even some tannins to dim the lighting if doable.

Pygmies can be really shy, ive had that same experience. In fact I barely see mine anymore. But they do come out at night.

Can also try keeping the tank in a spot that isn't too busy, so too much movement from people doesn't startle them.

[Shane]

First of all check for any mechanical issues. A noisy filter impeller, broken heater, lighting issues (brightness and or photoperiod) etc. Mechanical issues like these can cause fish to change their behavior.
Second, fish can be strange. I have a group of C. weitzmani that for six months I only saw at feeding time if I sat very still for several minutes. Now they are out eating and chasing each other even when I am making a racket in the fishroom. I have no idea what made them change their behavior.
-Shane

[medaka]

Now they are out eating and chasing each other even when I am making a racket in the fishroom. I have no idea what made them change their behavior.
-Shane

Maybe, it was because you stopped singing in your fish room?

[Sobe]

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 weeks ago, this one is now inaudible. Filter is also really silent. But yes maybe you're right and i should just be patient ?
@Medaka, i am singing a lot too , maybe this is part of the issue ! ^^

Anyway, I really feel concerned about their comfort and I am thinking preparing an other tank just for them in an other room where there will be no human or just, rarely.
And I will chose a less shy specy of cory for this one, because i really love catfish

Again, thanks a lot, and Have a nice day !

[Sobe]

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 !