Need help/advice

I'm a novice with fish and i've hit a problem. Back ground info is
I have a 30 gal tank that contained cichlids for little over a year. I got a
larger tank and moved the cichlids to the new tank after it fininished
cycling. I decided to try peacocks in the 30 gal tank. I got a batch of 6
peacocks. Within a day maybe two one developed a small white patch on its
upper fin. I moved to hospital tank and medicated as per lps with maracyn
two. It didnt make it. a couple of days after the first one was sick another
started up with the same thing. lps suggested to use PimaFix on the tank.
All in all i lost all but 2 fish. (one of them i moved to the larger tank
because he is very aggressive, now is seems to be doing fine) Also during
this time all the fish would just hide and only come out to eat. I waited a
little over 2 weeks and bought six more peacocks. (i tested the water before
buying and all was good - 0 ammon 0 nitrite 20 range of nitrate, water temp
78 F). I put these fish into the tank. All fish went into hiding for first
couple of days. When they started to come out i noticed that two of them had
the white cottony spots on them. One around lips, the other over right eye.
I pulled them out and put into hospital tank and med with maracyn as per
lps. They didnt make it. lps suggested to removed carbon from filter and med
main tank with PimaFix and to use some aquarium salt, which i did. Couple of
days after pulling out the two sick ones my pleco died. He was fine, eating
on side of tank and couple of hours later he was floating. Next day, another
peacock was dead. Checked water and 0 ammon/nitrite but with 80 nitrates.
Did a 50% water change and lps suggested to use Prime to knock down the
nitrate. I'm down to 3 in tank now and they all in hiding. Now i've also
noticed that on the sides of the tank there is very small spots of white
growths. I'm not sure what it is, could be algae but it looks more akin to
what was growing on the sick fish. I'm at a loss now. Is my tank infected?
Should i keep useing the PimaFix and Prime as a preventive? Should i just
wait for the rest to die off and empty/recycle tank?