What ails this Peace Lily?

I have several Peace Lilies having divided the original one many times. I have had them for years, and they used to be healthy, but problems may have started about 2 years ago when I used a drop of washing up liquid in the water I used to clean their leaves, and they started getting brown areas on the surface of the leaves that at a distance looks like mud, but isn't. Then recently I noticed that this one is getting small brown circular-ish spots on its leaves.

It might also be because I didn't feed them for a while and then gave them cheap house plant food, when previously when they were healthy they had Baby Bio or an expensive bonsai food somebody gave me, which contained a lot of trace elements and the Peace Lilies seemed to thrive on it.

These Peace Lilies have been the most tolerant of plants, but I am not good with house plants. I think it could be some sort of fungal infection, maybe that got into the leaves by damaging their waxy surface from using something other than plain water to clean the leaves with.

 

It looks like mould to me, maybe you wet the leaves too much.