Hiya im pretty sure the red and black finned top fish is actually a platy .it does look wasted ...its belly should b rounder than that.so as suggested before either quanrantine it and treat with an antiparasitic or cull.
Lovely female sword though.they naturally hav quite a round belly , are more aaggressive in nature, grow much larger than platies so the poor platy always ends up second best.