First, just to clarify (well, sort of...) the plant Nomaphila stricta
: According to the Kew Plant Index, this name is unresolved under the WCPS [World Checklist of Selected Plant Families], and probably refers to the same species as Hygrophila stricta
, which name is also unresolved. Unresolved means the names are neither accepted as valid distinct species, nor accepted as valid synonyms. In her book Aquarium Plants, the German botanist Christel Kasselmann gives both names as synonyms for Hygrophila corymbosa. This plant is in our profiles [click the shaded name] which has a brief mention of the foregoing. The above is somewhat pertinent, because it tells us something about the plant's requirements for light and nutrients.
To your question on CO2 and Excel. First, there is considerable CO2 produced naturally in a planted tank with a sizable fish population. The waste settles into the substrate as organics which bacteria break down, producing CO2; this is the main source, but is also supplement by respiration CO2 of fish, plants and bacteria themselves. If this natural CO2 is balanced by light and the other nutrients, the plants will grow fine. I am not familiar with the specified light, so can't say much other than I would consider this to be moderate, so the natural CO2 plus once a week dose of Flourish should balance fairly well. I would not consider CO2 addition necessary. And if the light is as I surmise, adding CO2 would achieve nothing without increasing the light and the other nutrients to balance.
And this brings me to the Excel and similar liquid carbon additives. I do not recommend them. Below is an excerpt from another site [just to give credit as it is not me] that sums this up.
Listed as the “Active Ingredient” on the bottle is Polycycloglutaracetal. Research reveals that, aside from SeaChem literature, there is no such thing. Polycycloglutaracetal, apparently, is a trade name for a product developed by SeaChem, which appears to be an isomeric form of glutaraldehyde. At least the Material Safety Data Sheet (MSDS) for Excel list glutaraldehyde as the active ingredient.
Glutaraldehyde is an antimicrobial, bactericide, fungicide, and virucide, commonly used to sterilize medical instruments. It is also used as an embalming fluid, as an ingredient in Anti-Freeze, an antibacterial agent in cooling towers, a leather tanning agent, a biocide in water treatment, a sanitary solution for portable toilets, and is used to sterilize ballast tanks in ships moving from one water source to another (to kill off pathogens and critters that may be transferred in the tanks from one water way to another).
Hope that is of some help.