The liquid in liquid CO2 is
glutaraldehyde. It is used a disinfectant for medical i tools. Apparently is it similar to a chemical in plants but I have not seen any scientific studies indicating it actually helps plants. The chemical is dangerous to fish if overdosed and does damage some plants. A long time ago i used it but didn't see any benefit. Most on this site don't recumbent it.
There are 2 ways to get CO2 information your aquarium:
- Use pressurized CO2
- aerate your water.
- More fish.
Aeration is basically a decorative water fall. increase water surface agitation or use an air stone and air pump to pump air through the water. Surface agitation involves water movement at the surface that is contact with the air. TYpically want enough movement to creat ripples on the surface . OR the output of the filter is sprayed onto the surface. The key idea is to maximize water contact with eh air.
For Pressurized CO2 many people use a CO2 diffuser but you have to be careful. To much CO2 in the water will kill fish and the CO2 output must be monitored daily and adjusted needed to prevent killing the fish. When I was Using this methode I had to refill my bottle once every 4 months. Best way to used use pressurized CO2 is to use
Passive CO2.
Passive CO2 is basically an inverted bottle filled with CO2. The pressure in the bottle and waters naturally ability to bind to the gas pulls it into the water. This is the most efficient way to get CO2 into the water. IT is also impossible to kill your fish with a Passive CO2 system. When I switched to PassiveCO2 my same bottle of CO2 can now go 18 months witout a refill.
Note there is no accurate way to measure CO2 in the water. Liquid test kits or strips are available but only work reliably with pure water. Normally all the additives in tap water,fish waist and fertilizer makes these tests inaccurate at best or comply unusable. Others use drop checkers or PH probes but these have the same issue.
The last way to get CO2 in the water is more fish. Fish and fish waste produce CO2 naturally. since you cannot measure the amount of CO2 produced by fish and the amount of CO2 needed it is impossible to say how many fish you need. But since you have a 36 gallon tank and the one inch of fish length per gallon recommendation many people use you might be able to get 30 small schooling fish in the tank with your plants. And the schooling behavior of the fish might be interesting to watch. Combined with good surface agitation you might get enough CO2 in the water.
Note the only indication of Low CO2 levels is slow growth. But in my experience lake of nutrients (fertilizer) is often the cause of plant growth problems. If the new growth of your plants doesn't look right or you have bad algae issues you likely have a plant nutrient issue. Just buying a fertilizer at the store is no guaranty that you will not have nutrient issues.