Platy red scratches

Tank size: 75 gal but relocated to 20 gal
pH: 8.2
ammonia: 0
nitrite:0
nitrate: 5 (75) 0 (20)
kH:??
gH: I think they said middle at the store last week. Master test kit doesnt test this
tank temp:78*

Fish Symptoms (include full description including lesion, color, location, fish behavior):
Noticed she was hiding a lot yesterday, today did not come up for feedings. Was concerned and noticed red "scratches" on stomach

Volume and Frequency of water changes:
Weekly/25-50%

Chemical Additives or Media in your tank:
Moved her to 20 gal and added melafix and salt

Tank inhabitants:
In 75 gal: 2 other platys 3 mollys and bristlenose
In 20 gal: 4 other platys

Recent additions to your tank (living or decoration):
Added live plants, 2 attached to rocks

Exposure to chemicals:
None
Digital photo (include if possible):

Is it on both sides of the fish?

It looks like something has latched onto her. The Bristlenose catfish is the only fish in the tank that would do that but they aren't renown for attacking live fish.

The other option is she got caught under something and rubbed it raw while trying to escape from whatever she was trapped by.

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Monitor the fish over the next few days and post more pics if it gets worse.

You have added salt and Melafix and that is fine and I would have suggested salt at a dose rate of 2-4 heaped tablespoons per 20 litres of water.

Do a 75% water change and gravel clean the substrate every couple of days to help keep disease organisms low. Add salt and Melafix to the new water before you add it to the tank.

Thank you Colin, it's only on the one side. I would be very surprised if our bristlenose attached to her so I'm thinking it is prob that she strached herself on something. I think I see a couple more red lines. I'll try to take another picture later this morning. She does seem more active and content in the 20 gal. Hopefully she eats today and continues to improve

Couldn't really get a good picture. This was my best attempt. Not actually much different from yesterday, although she is out and about more today. Now we have another platy in the 75gal thats acting very similar to how she was acting. Although no scratches that I could see.

Female platies will hide in when they are in labour or getting pestered by males. As long as the fish eat they should be fine. If they stop eating then there's an issue.