Sick guppy - turning white. Help!

I breed guppies in outdoor ponds. Heated year round. Started in Dec last year.

Yesterday my oldest and largest female died. Her Colour was almost all white. (normally a greenish body with yollow cobra tail fun).

The 3 days prior she was very sedative.

I wrote it off as one of those things.

Today a new mom I have is showing same behavior. Her underside of body also appear white. See vids and photo.

Advice on what's wrong and how to treat.

( notr: since Dec I only had about 3 fish in total die. Consider that I have probably >500 at any point in time).

Thank you

 

I should add that I added 3 new males from a well reputed aquarium shop in Jhb.

I didn't Isolate them. In hind sight, quite stupid...

2 of the 3 introduced went missing. Could be king fishers (bird) / dog drinking water / fish jump and dog got to it.

I do 10% water change daily with "old" water.

 

Hi there. Apologies for late reply.

I self am out sick.

Long story short - I took her to a pet shop owner friend. Prognosis : Fin rot.

I am treating the entire tuv with meds I got.

Let's hope all restores...