Sorry to hear about your pec (and the others of course). Apart from the aeration, which you've had good advice on, a couple of thoughts strike me:
I am assuming that the white spot struck after you had brought the plec home (as obviously nobody would be bringing an expensive wild caught fish into a tank with known health problems). Does this mean other fish were also new? You see what I'm getting at- could there have been an ammonia spike etc due to sudden overloading of the filter. Even the plec on his own might have caused it in such a small tank- I had a small ammonia spike when I moved my female bristlenose into my Rekord 70, and that tank was much more lightly stocked than yours.
Is the stocking list still relevant (apart from the dead fish you have mentioned)? If so, it does strike me as quite a heavy load on this tank- even assuming that the bala and the clowns are really small (which I am probably right in assuming). They are not massively filtered, these tanks; in fact, I am running an extra fluval on mine, together with the Juwel standard filter. I sort of wonder if your filtration is up to the job.