I had started using this, just a very small amount, in the drop trays of my smaller ones: http://www.amazon.com/So-Phresh-Pape...ords=so+phresh My bigger ones, I don't use anything in the trays, I wipe them out throughout the day. This is what I use to clean: http://www.homedepot.com/p/OdoBan-1-...YK9A&gclsrc=ds

None of my littles can get near their trays. I also have fleece on the floor over the grates.