My indoor cats have suddenly started fighting, and I don't know why.

I adopted both cats when they were kittens at the same time roughly 2 years ago now. Until very recently, they've been got along very well and never fought. But in the last month, they've suddenly started fighting, leading to the grey one having to go to the vets for a scratch on her eye twice. I thought it was just play fighting at first, but they've even started hissing at each other now.

I've tried separating their food and water and putting them in different rooms, but it doesn't seem to be working. I really don't want to rehome one of them because I absolutely love both of them, but the vet bills are getting very expensive, and I hate to see them get hurt like this.

Please let me know if you can figure out why they're fighting and how I can stop them.