I did a fish in cycle, works fine if you use aquarium salts to soften the water, mix in a 1/4 cup of instant ocean per gallon (instead of 1/2 cup) buffer with live sand/ rocks, keep at least 3X the filtration needed and make sure that when you mix your water aerate the water till it is crystal clear before adding. Add 1 gallon of this saline solution per week (daily water changes of at least 25% are a necessity for the first month). After the first month increase to 2 gallons per week of saline solution and with your 15g tank they should be OK until they are around 3 inches a piece, I've had my puffer since he was a tiny fry. Your story sounds so close to mine.. I would NEVER bring them back to the evil walmart empire, they will die for sure. I guarantee if you post him on craigslist and take pretty pictures you will find an avid enthusiast to take care of him.