Are you familiar with the nitrification cycle? Article link in my signature has helpful information on using the BB / beneficial bacteria.
I didn't see a parameter for ammonia.
How often do you do pwc
and how much do you change?
The cloudy eyes usually is from poor water quality - could be from ammonia.
If you have done a at least 25% pwc
in the past week, do 2 more pwc
back to back of 25-30%. Then the next day do another one and another the next day.
If you have not done a pwc
lately, over 8+ days, then do 15% then another 15% back to back (approximate % is fine).
Wait another 8 hours and do a 25%, then another 8 hours another 25%. Then the next day another 25%.
If you have done a 50% pwc
in the past week, do back to back - so 2- 25-30% next day the same, and so a bout 5 water changes.
The idea behind this is a modest change in water parameters, so that the changes are gradual and more gentle, but urgency is also dependent upon the degree of illness of the fish.
If it seems you might loose this fish you might want to speed up the water changes closer in time. It is hard to guess what exactly it is. Getting the water quality in top condition is essential for wellness.
It's certainly possible the bullying fish caused the weakened immune system, maybe illlness, like shared some parasites.
If you have a vac, you might use it to clean up the substrate, as a preventative measure in case of parasites.
Have you noticed the fish's poop? Color, white, stringy?????
I am sorry your cute Ranchu is having these issues. Hoping this can help get him back to health / figure out what is going on.
This will help get a good refresh of tank water in the tank.
Sometimes this is enough.