Top Fin Decorations

I reported problems earlier with my fish (local, freshwater, wild fish)
flashing against various surfaces even though the water parameters appear
okay. At that time, I also pointed out that the problem seems noticeably
worse after cleaning the tank, which consists of vacuuming the gravel and
replacing the water removed.

Now, I'm noticing that when I clean the tank and temporarily pull out the
decorations that they really smell. They smell like a mix between the black
mud you find in ponds, and gasoline. I've got three large decorations in a
125 gallon tank and they all seem to smell about the same.

It could simply be that waste was trapped under or within these decorations.
But I can't help but wonder if there was some chemical reaction that was
helping to irritate my fish. I realize Top Fin is a cheap brand, but they
sell this stuff by the truckload and I would certainly have expected that
they were safe for fish.

Anyone have any thoughts on this?

Jonathan