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There are a few ways to get CO2 for your aquarium. Some shops let you swap your empty cylinder for a filled cylinder, some refill your cylinder on the spot, and others help you send your cylinder to be refilled and let you collect it a few days later. Some hobbyists go to the gas suppliers directly for an on-the-spot or collect-later refill. Many of those who get their cylinder refilled wonder if they are getting back a full cylinder. This article will teach you how to tell.
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