I have been asking this exact question for a long time now. Aspen really does just fine in my opinion. Pretty much any loose substrate like aspen, coconut, cypress etc, will get into the water, especially if it is wet and sticks to the snake as he/she moves over the bowl. It is pretty much unavoidable. Aspen (where I live) is literally impossible to find in bulk so for me it is stupid expensive so I am one of those paper wasters and use unprinted news paper as it is super cheap, but if I could get bulk (or cheap(er)) aspen that is the way I would go.