The Polyfilter cubes I used were small enough to go through the neck of the plastic Pepsi bottles and for taking the used cubes out, that depended on where I was. If I had an apisto from a whitewater locality and I was in a blackwater habitat, I'd just use tweezers to get the Polyfilter out and put a fresh one in. If I were in the same area as the water in the bottle, I did a complete water change. I mail ordered my Polyfilter from That Fish Place in Pennsylvania. After getting it cut up into neat little cubes, I put it in an air tight zip-lock type bag to keep it from absorbing household chemicals. This technique worked really well in Brazil. Breathable bags were all the rage in 1998 when Lee Newman and I went to Peru, so I didn't take any of the Pepsi bottles down. The breathable bags were even better at holding Apistos until it was time to ship. Unfortunately, fish can get caught in the corners of these bags and get crushed. Ideally, I'd like to use the Pepsi bottle plus Polyfilter and flush the headspace of the bottle with oxygen - I wonder if I can buy a can of oxygen anywhere - in a spray paint/household cleaner/WD-40 size can.