I've got discus and serious algae (the black beard stuff), have been advised to go 1 1/2 doses each time and see if that helps or even try double.
Told to watch the discus, if they start coming up for air its too much for them and do a quick water change. Mine are fine at 1 1/2 x the dose but unfortunately so is my algae.
Whats another name for the rusty plec.