Red pest is a disease that affects fish and that causes red streaks on the fins, tail and body of the fish. Red streaks from red pest can occur on any of these areas of the fish and the appearance of red streaks on any part of the fish is a likely diagnosis of red pest; it will sometimes only affect the body or fins. The disease can in severe cases cause the fins and the tail to rot away. Red pest disease can infect a healthy fish but is much more common in fish with other health issues.
Red pest is an internal fish infection and external medications are therefore seldom effective. Although there have been reports of external antibiotics working in very mild cases of red pest, it is always recommended to give antibiotics internally. The best way to treat red pest is to feed the fish food dosed with antibiotics. Mix antibiotics with flake food carefully. Adding about 1% antibiotics to the food is ideal but a slightly higher dosage can be required in severe cases. Avoid using stronger doses if the case isn't severe as it can have unwanted side effects. It can be good to withhold non-medicated food before feeding them the medicated flake food to make sure that they eat it. There are several different antibiotics that can be used, such as tetracycline and chloromycetin (chloramphenicol). Antibiotics can be added to other foods like frozen food as well in the same concentration. If the fish is too sick to ingest the antibiotics you can try to add no more than 10 mg of antibiotics per litre water. This will however not be as effective and might not cure the disease.
It is also recommended to clean the aquarium and adding a fish safe disinfectant such as acriflavine (trypaflavine) or monacrin (monoaminoacridine) to the aquarium if red pest is present. Use 1 ml disinfectant per litre water. Stop using the disinfectant if the fish seem to experience discomfort by its presence. The above mentioned disinfectants will color the water. Disinfectants can be enough to treat the fish in mild cases of red pest but I strongly recommend that you don't rely on that. Also be sure to check all your water parameters to make sure water quality isn't a contributing factor. If the water quality is poor, take appropriate steps to remedy the water problems.