Hi,
We still need more information how this issue happened.
Water parameters (ammonia, nitrite, nitrate, pH)
Foods
Tank maintenance regimen
Tankmates
Tank size
When did you last buy a new fish? Was it quarantined?
Regardless of what may have caused this issue, I would suggest that the fish be separated in its own tank immediately. Anything that involves injuries exposed to open environment should be separated immediately as there are various factors that tend to aggravate the issues.
For instance, a fish has an open wound. Some people tend to think by leaving the fish in the main tank with an open wound while convincing themselves the increased frequency water changes will help, they tend to overlook the fact some fish are tempted to keep nitpicking the wound until the wound becomes severely infected. Anyone in contact with the wound may also suffer the consequences by contracting bacterial infectin resulting in mouth rot due to contact on an infected wound.
In your case, leaving that fish there opens the possibility your other fish may suffer even minor issues that you would not want in the first place if you were suspecting yourself the eye issue was because of a tankmate that harassed it overnight.
The hospital tank does not have to be quite fancy. A heater, a sponge filter and keeping it bare is all you need. Do not switch on the light for several hours except during observation periods only. This will minimize stress thus allowing the fish to recover much more quickly.
Aside from that, we have to assume eye flukes may be involved because most of the time, that is the cause of the bubble that is seemingly trapped in the lens and I assume you may not have the microscope to figure this issue more accurately. Get praziquantel and do a treatment for at least a week with a dosage of 5 mg or mL per liter if tablet or liquid form respectively. You can buy it as Prazipro or Droncit in petstores/online fish sites and veterinary clinics respectively. Be very careful not to overdose. While praziquantel is a mild treatment and will not disrupt the nitrogen cycle process, your fish may not have a tolerance for praziquantel overdose. On the last note, praziquantel is not a cheap med but it is worthwhile for use as it is very effective against flukes and flagellates.
Hope this helps.
