High fin stressed?

[Cissafur]

Hi I'm new to the fishy world, I set up the tank for my kids. I have a 20 gallon tank with plagues. There's been some losses in the past 5 months but overall pretty stable. I added a high fin female to improve my male to female ratio and from day 1 she's been a little odd. I'm wondering if its stress? She tends to keep to herself and can often be found in a corner with her fins tucked in nose to the top of the water. She's hasn't really grown much and seems small. When's has a weird bald looking patch on her head. Not sure what to do with her. There are currently 2 adult males, and 2 adult females (including her), and possibly about 2-5 fry...oh and a Cory cat.

 

[Cissafur]

 

[Cissafur]

Could She be a dwarf platy? Could that explain her size and lacking pregnancy?

 

[fluttermoth]

True dwarf platies have very little colour, and are very rarely seen. I suspect yours is either a runt, or unhealthy in some other way. She certainly doesn't look at all well.

Your tank looks very bare; it will leave most fish stressed if they don't have enough hiding places. I should also point out that corydoras are very, very social fish and should never, ever be kept without others of their own species.

 

[Cissafur]

True dwarf platies have very little colour, and are very rarely seen. I suspect yours is either a runt, or unhealthy in some other way. She certainly doesn't look at all well.

Your tank looks very bare; it will leave most fish stressed if they don't have enough hiding places. I should also point out that corydoras are very, very social fish and should never, ever be kept without others of their own species.

The tank isn't to bare, the pictures are from the top of the water but I Could certainly get more plants. Do you have any recommendations/advice for my dear runt? More plants and perhaps another female to relieve some of the male attention?
I did have another cory but one day recently it started swimming crazy the next day was dead?