I've used zip ties to seal the bag. As far as bypass, I have enough purigen to create a layer about a 1/2" thick in the tray that I'm keeping it in. I have 500ml in a fluval 205 which is a lot more than needed but the price difference between 250ml and 500ml wasn't much so I just went 500ml. I also use 1 liter in an ac110 on another tank and its in a fine media bag with a Velcro seal, this completely covers the sponge a out 1/2" thick.