Black neon tetra, Hyphessobrycon herbertaxelrodi
Although this looks very serious, fish aren't people and the loss of an eye for a fish in captivity isn't really all that much of a handicap. It would be more serious for a wild fish which still had to avoid predators and find food.
Initially the site will be very open to infection and this must be prevented, such a large open wound will also cause the fish osmoregulatory problems. Salt added at 3gms/l will address both these problems.
The fish may also be in a little shock and could also be bullied so isolating the fish in it's own tank until it has settled down and healing is under way will greatly improve its chances of recover.
Obviously fish don't habitually lose an eye, there is a reason for it. Quite often one eyed piranhas are seen, piranhas are quite aggressive when feeding and it isn't uncommon for a shoal member to be bitten by mistake or to come second in a fight. Some fish when packed with corydoras for travelling will damage an eye on the pectoral spines of a corydoras, again large numbers of imported corydoras have damaged or missing eyes.
If you find this in your aquarium it is important to discover the cause in order to prevent a recurrence. One common cause is Loaches! they seem to have the ability to remove the eyes of other fish for food. It isn't very common but if you are stumped when looking for the cause and you keep loaches in the tank it could mean that you have a rogue loach.
Occasionally you will come across a fish with a split fin or a split tail if the fish is otherwise quite healthy then do nothing. The split will heal, normally with in a couple of days or so.
Fish do like us occasionally scrape against things and end up with small scratches and grazes. These are nothing to be concerned about because an otherwise healthy fish will heal untreated. If this is a common occurrence just check that there is nothing to sharp in the tank which is causing the injuries.
This is slightly more serious because fish don't normally lose scales for nothing, it could be a sign of bullying or something causing the fish to panic and the cause needs to be addressed. If it is just one or two scales again the best treatment is to do nothing. Adding treatments unnecessarily or inappropriately will not help and could make things worse.
If the wound is deeper or a lot of scales are missing then treating with a general aquarium antiseptic will help to prevent secondary infections and healing will start naturally if infection is kept at bay.
If you find a fish which has jumped from the aquarium and is still alive it will be in quite a sorry state. It may have all of the previous conditions caused through flapping about on the floor and it will almost certainly be hypoxic.
Return it immediately to a bowl with some clean fresh water which is at the right temperature. Next follow these instructions: Hypoxia this has to take priority over all other injuries because this is the most immediate threat to the fish's life.
Keep the fish apart from other fish for the time being because in it's poor state it will be bullied and unable to defend itself. Once the fish is breathing normally add some pre dissolved salt to the tank at the rate of 3gms per litre. This will help to prevent secondary infections and help with osmosis.
Provided that the fish was found in time it should fully recover with in a week or so. If on the other hand it went to long without oxygen it may have suffered brain damage and will never recover depending on the severity the best option may be to end the suffering of a fish which can't feed properly ect.