as a backup to a scuba system connected to tanks, does anybody carry spare airs with them. They are small, but could give some valuable breaths if needed.

YES, We have boat mounted tanks with normal regulators as well as each crew member a spare air on their leg.

I have 2 spare air tanks with regulators for sale.just send me a PM

Yes we have regular bottle air and also 2 spare air's.

What size spare air tanks do you have for sale?

Model 300,3.0 cubic feet,3000psi,with regulators

Pretty sure Spare only make the one size.

We ran a separate tank for the driver and throttleman. This gave us each 30mins in a stressed condition and a back up in case something happened to one tank. We also had 20 feet of hose so we could swim out with air.

Nope, name brand "Spare Air" comes in 1.7 and 3.0
Some other brands offer a few different ones up to a pony

Awesome good to know I have only seen the 3.0