aspen gets smelly.. I try to change it out before it smells but sometimes you don't catch it. I smell each tub when my rack starts to smell, you can find the culprit pretty quick with this method. I don't have any proof but I think its a combination of high humidity and heat causing the aspen to mold. It could also be missed urine, if it dries before I find it? I spot check each tub minimum once a day and I still get the smell.