CO2 system means they have a system that injects CO2 into the tank to make Carbon © available to plants for use in photosynthesis. there are 2 types that are mainly used:
fermentation - the mixture of yeast, water, and sugar to produce CO2 (and alcohol as a by product), usually made as a
DIY kit
pressurized - this is a 'proper' system where you can regulate how much CO2 is injected, when to turn it off etc. You can uy kits that have all the parts available or you can put together a kit using a fire extinghuisher (see 2 guides in my sig)
For amore in depth explanation on CO2 please read here:
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