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Fish Medical Guide: Diagnosing Symptoms

Sores
Definition: Red sores are best diagnosed by knowing the fish's history. If the fish has suffered trauma the red sores are actual wounds, but if the sores slowly appear they are likely caused by bacteria and are developing ulcers. Redness in the mouth area is probably flex.
Causes and Treatment:
-Ulcers: Red sores on the body are often signs of a bacterial infection and ulcers. The bacteria that causes these sores is often Aeromonas hydrophila. This type of infection is often secondary to fluke infestation, so we recommend treating for flukes as well as addressing the sores directly. It is important to isolate the fish in a hospital tank. You can treat topically as explained here and combine this treatment with feeding of medicated food. Medi-Gold and MediKoi are the best choices for treating this kind of infection. If there is a white center developing, it is clearly infected and may be ulcerating. The fish may need to be injected with an antibiotic, or if that is not possible treated in a bath of Tricide Neo (sometimes sold as Neocide 3).This is a serious illness, but treatable, so please post in our forum so we can help you more specifically.
-Cuts and Scrapes: If the sore is the result of an injury, the best thing to do is to keep the water clean and feed good food so that the fish will nicely recover on his own. The sore can be swabbed one time with Iodine or 3% hydrogen peroxide. Additional swabs will kill the new, healthy cells growing in. If the fish has suffered some trauma and is bleeding, pressure can be applied to the wound to stop the bleeding. You may want to sedate the fish first as described here.
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Red stripes or streaks
Definition: Red streaks/stripes/veins in the fins, often the tail fin.
Causes and Treatment:
-Water Quality: Streak are often caused by poor water quality. Check your water parameters and perform water changes if necessary. These tests are best done with your own testers. As a minimum you need to test for ammonia, nitrite, nitrate, and pH. If you need help interpreting the tests, please visit GAB's Water Quality Forum. Some people associate red streaks with either high nitrate or with low oxygen. In either case a partial water change should help.
- Natural: On some fish (goldfish in particular) that are very light in colour, you can see bloodlines on their tails. This seems to be just a natural thing so if keeping nitrates low (under 40) and having plenty of aeration do not help, and the fish is acting healthy otherwise, consider these streaks normal.
- Bacterial Infection: If attending to water is not helping and the fish is looking lethargic, it is likely a bacterial infection. Treat by feeding antibiotic food (Medi-gold preferably) or if the fish is not eating, use a full spectrum water antibiotic like kanamycin or a combination of Maracyn and Maracyn 2. If the fish is larger, you should consider injections.

http://thegab.org/Articles/DiagnosisChart.html