Here's the story, I set up an amazon biotope tank with sand, wood and anubias. NitrAte was 5ppm after a week so I added 3 corys, the following week 4 more cories and gradually the number became 13 or so. Now the problem, there are 3 sterbai corys in the tank, they exhibit redness on the gills, I wonder if this is normal because they swim well and eat well(altho they look thin). Also, one corydoras melini who had been looking lethargic and had a dark color tone grew some cotton wool stuff on the body, it looked beyond cure so i euthanised it. Now it seems that some of the other corys are also showing milder signs of cotton wool growth(corydoras weitzmani,another corydoras melini) and all of them seem to be scratching VERY often.
I did a huge(80%) w/c and the fish seemed better. Can anyone help diagnose what's the problem? It seems that something is imbalanced in this tank, the occupants are about 12 corys and 1 two spot catfish. Could it be that the disease or whatsoever was introduced by the two spot catfish? because I was treating it for finrot before it was added into the tank.
I thought of adding salt to the tank but corys are known to be sensitive to salt so I'm feeling hesitant. Does anyone has any thoughts on what I should do? Only thing I'm doing right now is 30% w/c every other day.
Information on tank: 20g, internal filtration system + eheim pickup to provide current, 50% w/c weekly, ammonia 0ppm, NitrIte 0ppm, NitrAte 10ppm, pH below 6.0(I'm using the API test kit so the lowest pH value is 6.0) I'm not sure of the actual pH