black moor has bloody eye

I woke up this morning and was doing my regular fish check. My black moors
eye is totally red. It looks like it is filled with blood but I am not
sure. What could be causing this and how do I fix it. The tanks
paramenters are fine, I tested this morning. I have had this tank set up
for almost 7 months now with these same fish and have had absolutely no
problems so far.

Thanks, Sarah