Stopped eating
Adults can go a long time without food and recover. If one stops, check the water, then the diet: herring and anchovy destroy vitamin B1, and a redtail short of it refuses food and loses its balance. Signs and fix.
Low oxygen
A redtail in low oxygen may not show it, eating normally until it stops digesting and dies. Aerate generously.
Fin and tail injuries
Torn fins usually heal. Infection at the base of the tail is different: it can be surprisingly dangerous, and should be treated quickly, for example by dabbing iodine gel or undiluted acriflavine on it with the fish netted.
New arrivals
A newly moved fish is stressed, and treating it straight away can kill it. Quarantine in a large tub or kiddie pool, wait a day or two, and only then treat.
Cold water
Redtails survive 17 to 18°C but eat less and get sick more. Keep them in the 70s Fahrenheit.