A long time ago in a galaxy somewhere south of Malibu and north of the Repubic of Santa Monica...
My parents had a book called Exotic Aquarium Fishes
by William T. Innes, © 1934. As a child I was facinated by the color plates (drawings) of the "exotic" fish. I think we started out with a 5g tank and guppies. Eventually, we had four aquariums running at one time. One of my favorites was a brackish tank with an archer fish (Toxotes jaculator), a Scat (Scatphagus argus), and a Monodactylus argenteus.
When I was in the first year of high school a neighbor suggested I try a salt water aquarium and offered to show me how. He took me to some tide pools in Malibu and we waded out a little ways. He started turning over rocks, but didn't tell me what he was looking for. BTW, this was legal then. It isn't now.
Suddenly he started chasing something and came up with a small octopus in the palm of his hand. We took it home and I kept it for about three months. Fed it live crabs caught at the beach. When I got tired of that I released it back into the ocean. I managed to get a photo of it about to pounce on a crab. This is the only version I still have. The background was knocked out in Photoshop. Not sure what happened to the negative. It was amazing to watch it tiptoe up to the crab and then pounce on it. All that was left of the crab was little bits of shell being expelled from its exhale tube.
Later in high school cars, sailing, and photography superseded aquariums.
The technology has changed a lot since those days, but yeast-driven CO2 was being done back then. The only tests I recall having were pH test strips and a hydrometer.