You should not go by that "low light" tag they put on that
Hey, I put that "Low Light " designation on there! :-]
Low light, medium light, and high light are all subjective terms. We should be using actual PAR numbers so there will be less confusion. That being said, CO2 comes into play as well.... A tank heavily saturated with injected CO2 can grow the same plant in significantly lower light than in a non-CO2 tank. You also need to consider the type of plants being grown.
Back when I made that info-graphic showing CFL and PAR ranges, I was growing glosso, HC, and ludwigia repens in non-CO2 tanks. Anything lower than 60 PAR, the HC and glosso would not spread horizontally and it would get leggy. And the ludwigia would not turn red... All this points to the PAR being too low for the expression I wanted for those plants. Will glosso and HC "survive" in 30 PAR without injected CO2? Maybe, but it won't look good. The full expression of the plant requires more light intensity.
Also, in natural streams, when I measured where vigorous aquatic plants were growing, the PAR averaged 120-200 PAR at the stream bed, on an average day....
Those two observations led me to the original "ranges" I came up with. Are they perfect and all-inclusive for all aquatic plants? No. But in nature, 60-70 PAR is definitely not high light. In subjective terms, I would say it's at the "high-end of low light, verging on medium", for non-CO2 systems.
Adding CO2 changes the game a bit.... With 40-60ppm CO2, I can grow some carpeting plants in just 30-40 PAR. At the same time, typically higher-light plants will "survive" in just 15 PAR when the water is saturated with CO2. It comes down to light gathering energy versus carbon fixing energy. When you don't supplement with CO2, your light ranges will have to be higher to get the same expression of the plant.
Even mosses, ferns, anubias, and crypts benefit from slightly higher light levels, even though they will "survive" in very low light of just 15 PAR. But I bet you they would look a lot better grown in 40-50 PAR.
It's hard to put generalized ranges of light intensity on aquatic plants. It's best to give actual PAR values and describe whether the plant is reaching its full potential - and whether that expression is aesthetically pleasing or not.
It's one thing to make charts showing generic PAR ranges... And it's another to actually measure PAR on a daily basis, in real world settings, and make observations of the plants and how they are growing.
If a person is ok with a plant just "surviving" where its net-growth is practically zero because the energy it gets just balances the energy it takes to gather light and fix carbon, then I suspect you could talk about very low PAR values, indeed. But people usually are looking for a better expression of the plant. And in a non-CO2 tank, the lighting intensity has to be a bit higher.
All that being said, I suspect the original poster will have low light...probably in the 30-40 PAR range, using a 15 Watt CFL bulb mounted horizontally in a cramped housing with a lot of "restrike" off whatever is serving as the reflector. It would be better to find another fixture with a wider parabolic housing in which the bulb is mounted vertically (desk lamp or something). This would allow the fixture to be raised from the water surface a bit and allow the light pattern to spread some. Doing that, would allow the use of the same 15watt bulb and put the tank in a better lighting range for most common plants.