How about a 10 lb bottle with a regulator at a local flea market for $25? Not sure what is it was used for, it has a 120v solenoid attached to the reg. any ideas?
most likely it was being used to supply supplemental co2 to growing plants, it's pretty common with hydroponic growers who grow indoors. it's also commonly used in aquariums that are planted with live plants. the regulator is plumbed to emitters and the solenoid is connected to an interval timer.
$25 is a good deal, even if the tank is out of hydro.