if you only have plecs then in theory it can,just make sure youve got plenty of oxygen in there and you dont overdose and things should be fine.
If you have any scaleless fish (loach etc) the dose should be halved,and i would think seriously about even using it

seek other treatments for it such as anti slime and velvet (a uk product,not sure what you have in singapore)
As regards to the dosage,follow the instructions on the bottle and you should hit the nail on the head
