this seems a common problem with the BN plec. I have a few theories, but most plausible is there's something rotting in the tank, both occasions my BN has had this, i later discovered rotting matter, once it was a algae wafer that got lodged under the filter, the other was a small fish that died and somehow got under the substrate (i count the fish regular, but somehow miscounted and assumed i was ok). So basically, do a water test (look for nitrate/nitrites), and above all, do some frequent water changes. go 50% day 1, 50% day 2, 25% day 3, 25% day 5, then keep do a few more every few days. This has worked for me, the white spot goes within 7 days.
