Help! My terrarium stinks!
 

I bought a small aquarium a few weeks ago and put some rocks in it along with mounted and potted orchids. I noticed a sour smell a couple of days ago and this morning, when I lifted a few of my plants, I see this scum on top of the water (pic 1)!!

Of course, I am going to immediately dismantle everything, soak the rocks and the aquarium in bleach and start over.

But . . . what could have caused this? I don't want to make the same mistake. Most of the rocks are just bagged from a craft store. A few are from a rock pile outside, which I bleached.

In pic 2, you can see the water level is not high. In smaller terrariums, I've always let water stand in the bottom. It stays the same level due to evaporation and watering cycle. Is that not good?

Any ideas?

My only thought is that it could be the standing water. I know if you have water without a bubbler it gets stagnant and stinks, especially in enclosed areas like tanks. And if there's no fan to circulate the air it adds to the problem.

Even if my thoughts aren't right, this will give you a bump and hopefully someone with more experience can help.

Thanks nikkik!