Durbkat... looks like some people round here cant read....
just from what i have read it makes perfect sense....
you plec is sat on the gravel with his tail end 1/4" above the gravel.... whats hard to understand about that....
that sounds normal for a plec - wait till you find him nose first in a plant with his tail pointing towards the sky
as for the white spots / patches, are they raised or clearly above the slime coat? is so it will probubly be white spot and you should treat ASAP with a white spot treatment. If you cant see anything raised then it is most likely just colouration but keep a close eye on it.
as for tank size... 20gal is larger than our common started life in... but either take him back or get a bigger tank by the time he gets to around 6"
hope this helps.