It is a slime-coat reaction to a sudden change in water chemistry. The stores pH/hardness/nitrates to your home tanks pH/hardness/nitrates. I doubt that it is a fungus that developed overnight. As far as the silica goes, his underside will be all cut-up if the silica is an issue, he will be lifting his disc off of the bottom to avoid further irritation. If this is the case set-up another bare-bottom tank for him. Or not, I mean he only cost $39, so why bother right?!? If he dies it is b/c he is a retic, you did "save him" from that 20 gal already...
-Like nic recommended, increase water changes, but also test your water to get a base reading now, and then again after water changes.