my balla sharks died within 1day of a normal water change

Question
hi , i have a 30 gallon tank , it has been setup for about 8 months . I had 2 tiger oscars , and 2 ballas sharks .
I regularly clean the tank every 3-4 weeks , filter , 50%water change , stress coat and dechlorinater. I have been doing this since i got them .when i got home i discoverd one fish dead and one in the bottom corner of the tank breating heavily , both had many scales missing and what seems to be black dots coverg the exposed areas.the tiger oscars seem to be fine .i also have a amonia meter but nothng has even registred on it . im mystafied to what could cause this. your helo would be greatly appreciated.

Answer
Hello Shane,
The Fish probably have ick. Oscars are very tough fish and have a great immune system to put up with the deseases like ick. Balla Sharks on the other hand are not. They could be both males and could be fighting, therefore causing themselves to be in high stress levels. Stress, and frequent water temp. changes, all lead to ick. I use Rid Ick + as it is the best medition i have ever used for ick. It seems to clear the sickness up quickly.
Hope This Helps
Kyle