Do the blotches look paler than the surrounding color? If so, you might want to consider whether the problem is ich. My pleco died from ich about two months ago, but I didn't realize the problem because he didn't look have the trypical "grains of salt stuck on your fish" look. He was the first in the tank to succomb, so I didn't know what the problem was until the day he died, when several other fish started to look, well, like they had grains of salt stuck on them. The pleco, which was black with gold spots, just started to look more washed out and grayer in areas. I have my theories on why he looked different from the other fish, but regardless, if you've added someone or something to the tank lately, you might want to consider ich.