Updates... Fungus/Columnaris/other...
Well, I lost almost all my female guppies, only two left. I treated the tank with Tetra Fungus Guard (for 3 days now) , but it's doesn't seem to work. I think I may have a COLUMNARIS problem, I don't know.... Each fish had different symptoms. Some with little cottony growths at the edges of fins, two with internal bleeding, one with an open red sore, another with mouth rot. I had a similar problem some months ago, and it wiped out almost all my guppies (always the guppies...).
All my water parameters were fine, the only thing I think of is that could have been introduced in the tank with some new fish or shrimps I bought last week.
I have other fishs in the tank, and I didn't lose any of them : one male betta, a little microrasbora galaxy, 2 female platys, 2 otos, 1 cory panda, 3 cherry shrimps, and a couple of dwarf cory. I even have about 15 guppy and platy fry (some only days old), and all seems in perfect health.
I will wait till the end of the treatment with FUNGUS GUARD, do a good water change, and maybe start another treatment with API TETRACYCLINE, to be sure to eraticate the problem.