Amazon offers its Prime members the Seachem Purigen 100ml Bagged Aquarium Filtration Kit, model no. 67101650, for $6.70 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.01.) That's the lowest total price we could find by $2. This regenerable filtration product can be used with marine and freshwater tanks, treats up to 100 gallons, and controls ammonia, nitrites, and nitrates.