Just some side questions: Is the lid totally closed or is there some gap for the betta to breath? What type of substrate are you using because it looks really nice.
There is a gap in the back. I might put something else in there to make it bigger but it is sufficient for now. With the light getting so hot, I sometimes completely remove the lid in order to keep the water from overheating. I don't like to do that for fear the betta would jump, but I have no choice sometimes.
The substrate is Soilmaster Select, a clay based material used in baseball fields and such to keep things from getting too muddy! I paid $15 for a 50 pound bag, and since the material is lighter than gravel, one bag ended up being enough for my 55 gallon tank with a lot left over.
Dosing flourish sounds like a good plan to me. However, I think the main "culprit" is the incandescent lighting...the Kelvin rating does matter.
I think the culprit is the lighting, and now that I see your setup, this can easily be fixed. Many desk lamps can be fitted with compact fluorescents, which will give you the wattage that you need. I'm also a little concerned about gas exchange since it's a closed system.
I had considered the compact flourescents but I can't find any bulbs that don't stick out the end of the lamp because with the little ballast, they are pretty long. Plus, I think the lamp is junking up my counter. I am considering rigging something to mount under my cabinet for it, using compact flourescent bulbs.
My money is on the lighting too..... under my 3.6 WPG my Anubias' were suffering from much the same problem. They recovered quickly once moved to a shady spot. They're hardy plants but too much light will kill them way quicker than too little. A liquid fert may help....Tropica Mastergrow is good but may trigger algae outbreaks (i've yet to prove this one!)
So you think it is too much lighting rather than too little? Maybe since it is so close to the light. I really don't know what is best! I could shade it with the hornwort for now, but would that help or hurt? I am not sure.
I haven't seen the tropica around here. Maybe I haven't been observant enough, but I'm not sure they have it.
EDIT: Next time I go to the hardware store I am going to get some different lighting for this thing. How many watts do you think I should go with if I use a screw in flourescent bulb in an incandescent fixture? I know the WPG rule goes out the window with nanos, but by how much should I overdo it?