Discus fish fin rot

thank you everybody for your help, I will keep you informed how it turns out. Hopefully I can post some photos of the tank when it looks its best, not its worst.
 
I lost the first discus last night, starting the tetracycline today, how long should I leave it in the tank before doing another water change, it doesn't state on the directions for use, I am guessing 7days. PH 7.1, ammonia .1 ppm, nitrite 0, nitrate 10 ppm, I have vacuumed the gravel and added another 9500 dual air pump while the tetracycline is in the tank.
 
Need to know: What are the dosage instructions ? How much daily, or otherwise ?
WCs should still be done - perhaps daily @ 25%, but not sure until I know the dosage instructions.
It's still a bad sign that you have ammonia traces, although the use of Prime should mitigate.
Sorry you lost 1 fish, hope you can save the others.
 
Dosage 1 tablet per 20 litres which is 30 tablets for my tank. I did a half dose and fish immediately seemed more stressed so I did another water change. Lost another 4 fish this morning ( very depressing) latest water readings are PH 7.2, ammonia 0, Nitrite 0, Nitrate 3 ppm. there is 1 more fish that is doomed but I still have hope for the rest.
 
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