Different species can react differently (sometimes disastrously) to different chemicals or conditions. E.g., although dogs and humans are both mammals (like Oscars and Clarias are both fish), dogs can die from eating garlic and chocolate which most humans can eat with no ill effect.
Because one fish appears to be fine, there is no reason to assume others aren't suffering.
And how did the RTC and Naja Shark die? Is it related to this or is it just a coincidence?
i got it you, i contacted pet store and asked them to keep my fishes with them and treat them, and of course i will pay them. lets see what happens, hope for the best
and naja died cz of no reason, i woke up one morning and saw it upside down at surface, it was all superb last night and was eating perfectly, i dont know reason behind that, and RTC died cz i didn't have any experience and i didnt know its nature, when i bought it the pet store owner told that it keeps on sitting on bottom and never comes up, but i guess when i bought it, it was already ill. it died 2 or 3 weeks after i bought it.