Hi, AirSong... I'd like to make a few comments on your tank and your quest for lighting. I keep small tanks like yours myself with good success - and I thought I'd share a few ideas with you and also dispel some half-true statements that are constantly being echoed among hobbyists.
I'm glad to see that you are targeting the light. This is very important as it is the one thing that drives the biology in your tank. Most people fail from the start by not really assessing the light in their tank. They like to apply rules like watts per gallon etc, but that is not really assessing the light.
Your original HC died because it wasn't getting enough light to photosynthesize. Even if it had been given enough light, it would have died because of lack of nutrients (given the picture in your last post).
People will tell you that you need injected co2 to grow HC, glosso, riccia etc. but it simply is not true. I grow them, and I grow them well in non-co2 tanks, they create nice carpets. But there's a lot of accumulated knowledge that goes into it and you can't just follow everyone's "rule of thumb" to achieve success. You have to put more attention into knowing about your light, and you have to know exactly what kind and how much nutrients you are providing. Then you will have a much better chance of success.
You say that you "like the low light, and anything more would be too much for your room". You can't use this a basis for your tank if you want to grow HC or glosso etc. I would also tell you to forget about anything you've heard about the "watts per gallon" rule. It's antiquated and it holds no meaning.
WPG rules would only work if everyone used the exact same kind of light, with the exact same kind of efficiency, placed exactly the same height above the same size tank. You've already encountered that the light you wanted to use didn't fall neatly into everyone's rule of thumb. Also, just because one person used 27watts of a certain bulb on a similar sized tank doesn't mean it's going to work in your situation... for many reasons.
What exactly is low light? What exactly is high light? The amount of watts isn't going to give you the answer. Here's an example: I have 100 watts of metal halide lighting above my 5gal tank (really). That comes out to be 20 watts per gallon. Is that high light? Or could it be low light?
Many would say that's definitely high light. But, the answer is that it is low light, given the way it is set up
. And you can only know that if you measure the light in a different way: a PAR meter. A PAR meter doesn't measure LUX or Lumens, those are human vision quantities and don't mean anything to plants. A PAR meter measures the amount of light that is available for photosynthesis. It has a funky unit of measurement because it counts the number of photons that strike a surface over time.
Now, I know you probably don't have a PAR meter. But, I do and I'd be interested in helping you determine exactly what bulb might give you enough light, given your fixture and distance from the water. I can tell you immediately that you're going to have to have more than 15watts of the bulb that has been mentioned earlier. If you're interested, send me a private message and we can work out the specifics, I have an idea.
After you get your light worked out, I would concentrate on the substrate next. Based on the picture of your tank, I would not consider that to be a adequate substrate on a couple of levels: one, it is too coarse for small rooted plants like HC and glosso; and second, it effectively is inert and provides no nutrients to the plants.
There are so many philosophies on what kind of substrate to use, but whatever philosophy you choose, you'll have better success if you choose one that actively provide nutrients to the plants, like soil for example.
Personally, I use a layer of yard soil in the bottom of all my tanks topped with a fine grain gravel - and I think is an important aspect of a non-co2 tank. Diana Walstad's book covers the mechanics of aquarium soil quite extensively, if not intensely.
I would highly recommend using real soil. Some claim it is messy, causes cloudiness, and is generally a pain - but I do not find this to be so. The hardest part is finding the CORRECT source of soil. And for small tanks it's nice because you don't need that much... only a 1/2 inch layer on the bottom.
But, even if you decide that soil is not in your interest, then at least use a fine grain clay fractured substrate: the fine grain helps tiny roots, the clay fractured aspect allows better bacteria colonization.
Especially if you don't use soil, you have to provide water column nutrients. Fish waste and uneaten food will provide some nutrients, but it is better to supply a consistent source; and more nutrients than you need (to a point). The idea is to not limit the nutrients to the plants. Once your plants become limited in any aspect, you are setting the stage for algae. So supply more than they need, and they will always be growing. Please don't fall into the trap of thinking that excess nitrates and phosphates causes algae. Read about Estimative Index to learn more.
Even in my non-co2 soil tanks, I dose 10ppm nitrates and 2ppm phosphates and 0.2ppm iron each week.
Whatever the method, you will have to actively sit and look at your setup and ask yourself: How are my plants getting nutrients?
In my opinion, light and nutrient deprivation is the BIGGEST reason people fail at growing HC, glosso, and riccia in non-co2 tanks. As far as nutrients go, this is why soil is good because it supplies a steady slow flow of nutrients to the plants.
In a very small tank, it is better to use your dry ferts to create dilute nutrient solutions that you store in bottles. Then you can dose small amounts of the solution into the tank. To create the solutions, all it takes is an accurate gram scale, storage bottles, a pipette for dosing, and a little bit of math.
Don't be afraid, unleash that inner chemist... at least a little.
Here are some pictures of my tanks... Can you tell me which one is non-co2?