I would probably put the
CO2 output under the filter input. This would allow any undisolved
CO2 to get sucked in a chopped up.
I'm partial to Rotala rotundifolia and Ludwigia brevipes for fast growing stems.
You'll need to measure you're
CO2 levels. The easiest way is to get a
CO2 drop checker and fill it with 4dKH reference solution and add the indicator drops. When it turns green you'll have 30ppm
CO2. The other way is to test your water after it has sat out for 24 hours or been aerated for an hour for both pH and
KH, then check the
CO2 chart to ensure that your results are about 3ppm. If so, then you can take a sample of aquarium water and test for pH and
KH and the results will tell you your
CO2 levels. If not, you won't be able to use the
CO2 chart because there is a buffer other than
KH present in your water. In this case your best bet is to get the drop checker, but you can also guesstimate by looking for a drop of 1 in the ph ie before
CO2 7.6, after
CO2 6.6.