Hmm, so I emailed Sargeant's pet supply who distributes Maracyn Two and they said that they only recommended using the powder in a 10 gallon tank, and not to split it. I don't know who is teaching customer service to have this kind of reply, but I have never heard of a 10 gallon hospital tank, and they do recommend using a hospital tank.
So, basically, this is this person's way of saying, "I, personally, don't know what the shelf life is of an opened packet, and no one has told me at my company." Or it may be that no one knows the shelf life.
But the idea that you need to use the entire packet is disturbing. If the powder does not divide easily, then that is a major problem. No where on the packaging does it say I NEED a 10 gallon tank to use the medication. The annoying thing is that instead of explaining, they just say, "Don't split it."
Thank you for contacting Sergeant's Pet Care Products. We appreciate hearing from our consumers and answering any questions you may have. Please be assured that quality products and customer satisfaction are our primary focus here at Sergeant’s Pet Care Products.
The shelf life of the Sentry AQ Mardel Maracyn Two is 2 years. There should be an EXP date stamped into the packets. This product is not labeled to be split or administered into a tank less than 10 gallons. Once a packet is opened, the entire packet should be administered into your aquarium.
If we can be of further assistance, please feel free to call us at 1-800-224-7387 or reply to this email. Thank you again for taking the time to contact Sergeant's Pet Care Products.
Sergeant's Pet Care Products
Consumer Affairs Department