Originally posted by Mantis_22
Do you need a CO2 device if you only say run 2-4 plants in a tank?
No.
Plants are largely made up of carbon and nitrogen. The figures that come to mind are 40% carbon and 30% nitrogen but they may be wrong. The point is that the plants need these things to build tissue. If you have only a few plants they'll be able to thrive with what's in there. As the plant load increases the demand for carbon increases, so folks add supplementary carbon.
There is a way around this: the natural lowlight planted tank.
If you keep the light modest (not insufficient), the plants can only grow to a given level. At this level the ambient carbon should be sufficient, especially if you let it generate on the substrate in the form of mulm. In nature a lot of the CO2 that the plants use comes from decomposing organic matter on the bottom. Lowlight folks feed extra food into modestly stocked tanks and don't scour the substrate with a gravel vac once a week. We tend to remove the carbon before it can cycle back in. Stuff like that. This is a low maintenance approach but you still need to pay attention to the rest of the ferts and be aware of what's going on. You could search for "low light" crossed with plantbrain to start. Like
here.
If you want stronger growth and more plants and go and add stronger lights then the lights are no longer a limiting factor. And maybe you've gone and added all your ferts as well. But now the plants don't have enough carbon. So they can't use all the ferts you've got floating around in there. Algae will take advantage of this imbalance. One rule of thumb I've heard is CO2 for any tank over 2wpg. (Its a rule of thumb, add a grain of salt… generalizations have exceptions).
Seachem Excel is an option for modest carbon supplementation.
DIY is a popular option. Or pressurized.
Lights, fertilizers (macro and micro), and CO2 all need to stay balanced with each other. Low light, modest ferts, no CO2 works fine. High, high, high works as well. High, high, low doesn't work. If you want to get all that working together in a coordinated manner, start
here
HTH
the carpfeller
