Hi guys I'm new in this forum, I need help, normally I would just read what u guys post here but then now I have a question that I'm puzzled with

I have this platy, the tank has been set up for more than a month and fully cycled

Since yesterday it start to sits on the bottom of the tank, it normally stays on the water surface, so I find this scary, the find are clamped, only flare when it was disturbed for example when I'm trying to catch it.
and since I'm new To platy I have difficulty to identify this one is a female or male
Most importantly is im scared it will die as it keep sitting on the bottom for almost 2 days.

In the tank
Sailfin platy x1
Cardinal tetra x4
Glass catfish x2
Red tail and head tetra x3
And 3 rasbora

Additional info would be that I'm using electric carbon dioxide nisso 900 for 2 ft tank a day before this happen seems like I injected too much CO2 as they were gasping on the surface but I did water change and aerate it straight once I saw which is like only 30 minutes cause I wanted to make my plants pearl but it failed too


I do have a photo but how do I upload it? Thanks