Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Fish food is usually high in N, P and traces, low in K, Fe and Ca.
Water with a GH of at least 3 degrees usually has enough Ca.
CSM+B has Fe, though some people feel it is not enough and add a little more.
There might be some K (potassium) deficiency, but I agree with zapins: Plecos rasp the leaves, eating the algae and microorganisms off the surface, and can do some damage to the leaves. Smaller Loricariads like Otos do not usually do so much damage, but Common Plecos can get large and can rasp pretty hard.
I think I would be dosing some K, such as K2SO4, just to be sure there is potassium. If the problem stops happening, then you know that lack of potassium was at least somewhat to blame.
Feed the Plecos more vegetables- fresh or cooked. Just no so mushy they fall apart in the tank.