Well you have an enormous bioload for a 20 gal
. That would be the primary cause most likely. Have you tested for ammo and nitrite? Large PWC
's are best at lowering NO3
. 25% or more at a time would be best. When doing larger PWC
's, you need to be even more careful to match up SG
, PH, etc. I would hurry and get the 120 up. That tang is not going to fare well for long in such a small environment, along w/ the water quality issues your seeing only getting worse.