First off how large is the tank... how much lr
and most important what type of water are you using. Tap, ro
?? have you tested for phosphates, silicates, nitrates..
Not sure how much you cut back on the lighting but this is only a temp fix.
If its red slime/cyno bacteria a Tang will not help. Do a search on cyno bacteria, cyno, red slime on the site and you will see tons of info on this.
Water quality, low flow and lighting are things that bring on cyno.
You need to have your water tested to find out whats going on...