Genereally used in the Aquarium industry these regulators have a few common features.
Many are Dual Gauge and some features solenoids. The functions odsolenoids are to enable on off cycles with timers or with the sms 122 ph controller by Milwaukee Instruments.
Some of these regulators come with bubble counters as well. The function of a bubble counter is to enable the user to see how many bubbles a second is going into a freshwater or saltwater aquarium. If you are using this for a greenhouse or other application you would be better suited with a regulator that has a flow meter.
For Freshwater aquarium applications the co2 is released into the water. Some sort of mixing needs to take place via a difussor or reactor. This increases the amount of gas in the water for the plants to take up for photosynthesis.
In a saltwater aquarium the gas is needed to increase the calcium in the tank. A seperate calcium reactor is needed to increase the availability of the calicum present in the saltwater in conjuntion with the co2.