Can you show me a picture and explain exactly how a Co2 system works in a aquarium?
Plants need a source of carbon to create energy for growth. There will usually be a small amount of CO2 in a tank that comes from the atmosphere which the plants will use.
When plants breathe in CO2, they will take the carbon from the CO2 and breathe out oxygen.
In an aquarium there are 3 main sources of CO2 for plants to use. From fish, chemical additives, and through co2 being diffused in the water.
The amount of co2 that fish create is generally enough for low light plants.
Chemical additives add carbon to the water that some plants can use. There isn't any oxygen attached to this kind of carbon so it doesn't help with oxygenation. Seachem's Excel is an example of this.
Diffused CO2 increases the amount of co2 in the water directly for plants to use. Either a DIY co2 system or Pressurized co2 system gives this.
Additional carbon sources are generally required for anything above a low light planted system.
As for the GPH of a filter, I feel that more is better unless your fish are getting thrown around by the current which is very difficult to do. I have a filter on my 55g which does approximately 4x the water volume per hour and I find that it doesn't create nearly enough flow in my tank.