Stinky Rainbarrel

Hey. I have several rainbarrels and I put mosquito dunks in them to keep out the mosquito larva.
One barrel in particular had a horrible odor and my daughter convenced me that their must be a dead animal at the bottom. It has no lid, so this is possible. (Just had 3 squirrels die in a small bucket of water, so now we are looking for screened lids.)
Anyway, I dumped 55 gallons of rainwater and no dead animal. I'm glad, but needed that water!
My question is this: What could be making the water smell so terrible?!
There was a very small bit of pine straw (maybe a handful) and some dirt at the bottom, but no algea or anything else. Not only am I wondering what could be causing the odor, how can I fix it? Are there any type of deodorant tabs or odor eaters out there for rainbarrels? With the growning number of people using them, I sure hope so.
Thanks for any help!