Hello there

I've just noticed that my two plecs are exhibiting signs of what I
believe to be some kind of disease.

They both have a white patch, one on the tail, one on the head, as if
they're losing scales.

I have uploaded a photo of the largest plec to the photos area "white
patch on plec". It's (the plec) around 3-4 years old, about 7-8ins
long.

The other younger leopard sailfin plec has a similar white patch on
it's head area.

The tank they're in is shared with 5 sunset mollies, a male angel
fish, a pair of sword tailed thingies (can't remember the species
name - they're red and black, the male having a protruding "sword" on
it's lower tail fin) and 7 silver tetras.

None of the fish are showing any signs of distress as yet, although
the large plec has begun to dash from on end of the tank to the
other, as if spooked.

The tank is 3ft x 18" x 24"

The sunset mollies were introduced to the tank relatively recently,
around 5-4 weeks ago.

The water chemistry tests normal for ammonia and nitrate.

If you could give me some advice it would be greatly appreciated.