Metricide 28 contains a surfactant which I believe is more or less like a soap that extends the effective period when used for sterilization of instruments. However I don't know if it's in the activator, which we don't use, or if it's in the 1 gal bottle. I would assume it's in the main solution. Nonetheless some have reported using it fairly long term with no issues.
There's also Metricide Plus 30 at 3.4% Glut but I believe it contains a surfactant also.
I don't have CO2 and dose 30 ml a day of undiluted Metricide 14 in my medium light heavily planted 50gal with great results. I've been able to buy in bulk of 4 gal cases at about $18 a gallon.