Hi, have used this stuff but only in my QT, I hope thats where you want to use it too,but first why do you want to use it? What do you think he has? If its ich ,you should do hypo in QT. I would advise against the copper, unless you use a QT tank and even then it should be last resort . Powder blues are very sensitive. Even a fresh water dip would be a good idea(same temp and PH as tank water). If other fish are dead, I dont think Pimafix is gonna fix the problem. How long have you had these fish, what are your tank parameters???