I used to use just carefresh in the little pet store cage that I had for my 2 cavies. Sad, yes I know...but I just made a 2x5 C&C cage and it's awesome!! Anywho, to use JUST carefresh now would get to be way too expensive (dont say anything along the lines that "if youre worried about price, and you cant provide the best for you cavies, then you should rethink having tehm"...well guess what? I HAVE THEM. theres nothing i can do) ...sorry about that lol... umm anywho, I'm now using a base of Aspen with a layer of Carefresh on top.. is that okay? I just put it down last night. on the bag of aspen it says "no aromatic oils" ... I assume that's good..if they're not lying. are they lying? thank you!