What can it be?

[paulbrear]

Last night we noticed that one of our female platies had a red mark on her side. Isn't behaving as 'ill' although she is spending time at the back and top. Have added a pic for you, thanks.

 

[dskidmore]

Cut? Infection? Parasite? Does your camera have a macro zoom setting so you can get a real close picture?

Test your water quality and do whatever you need to to keep the water clean. No matter what it is, clean water will make it easier to heal.