The JBL bottle is exactly the same as the Fire extinguisher from a safety perspective, both die cast aluminium extinguishers with EU regulated safety valves on them, you're just fitting a regulator to them in the same way, so there are no differences except with the FE you have to remove the horn in some instances which is easily done, just unscrew the nut with an adjustable spanner.
You measure the levels of CO2 using a drop checker, it's a piece of glass ware which you partially fill with 4DKH water and bromothyml blue low range PH indicator solution, it is then turned upside down and place in the aquarium so a bubble forums in part of it, as it is sitting in the aquarium, the CO2 in the water will diffuse into the air trapped in the drop checker as it has a lower concentration of CO2 than the water does, as it diffuses into the air chamber, it also diffuses into the 4dKH water with indicator solution, this changes the PH of the solution and it will change colour, you're aiming for a lime green colour, this is when the solution is at PH 6.6 and it indicates a concentration of 30PPM of CO2, if the solution goes yellow you've over dosed and risk harming the fish, if it is blue it isn't enough, which is why I recommend you start injecting low and then increase based on your findings, it should be noted that what you see in the drop checker reflects the CO2 levels approximately 2 hours ago.
