Ok Mand here's the scoop. I think your soil could be suspect. I know you said that you had used some store bought soil for at least 2 plants. How are they doing? Do they show the same discoloration? If not, I would be looking at my compost heap and wondering what was going on.

I use a mixture of a "SuperSoil"+perlite+vermiculite and have always had good results. I use the same mixture with all my house plants. I drains well and is and lets air into the roots well too. I've never been around compost so I don't know what the consistency of it would be like.

As far as water, I just use tap water.

I suppose if I were having a consistant problem I would have to have all of that checked.

Looking at one of your plants with 3 sets of leaves, 2 of the sets have the brown marks on them. The new set doesn't yet and hopefully won't. The plants look healthy, big leaves, ridge stems. There still pretty young.

Mand you might have to get all HighTech on us to figure this one out. I wouldn't put away your thermometer and fan just yet.

If the temperature isn't dropping below the 60's over night, your good there too.

I hope some of this helps.