Bro, I don't mean to be rude, but I will talk straight.
Please don't be offended, but I think you are doing everything wrong.
You are not cycling your tank, instead am changing water every week. So your bacteria has no chance to grow at all.
You are getting low ammonia , NO2, NO3 reading, is because of your water change. Not because the tank has finished cycling.
You added too much fishes at a time, and you chose fishes (tangs & butterflies) not suitable for 1.5ft tank size.
Not surprised the copperbands died, but surprised that tang has not come down with ICH yet.
You tank seems to be going through a diatom bloom now (or cyno, I do not know).
Would highly suggest you ask your friend to adopt the fishes for now (or sell them), while your tank goes through it proper cycle.
In the meantime, you can find a lot of useful info in this SRC site. And seek advise from friendly reefers here.
Cheers