Light is not light, and the mismatch is the most common reason bioactive planting fails while everything else goes well.
Your eyes are the problem. Human vision adapts over an enormous range and is most sensitive to the middle of the spectrum. A room that reads as bright to you can be a small fraction of what a plant needs, and you cannot perceive the difference because your eyes have quietly adjusted. Everybody says the tank looks bright. It is not a useful measurement.
Plants respond to photon count in the range they can use, which is a different quantity from apparent brightness. A fixture sold on lumens is optimised for looking bright to you, not for driving photosynthesis.
Distance punishes you harder than you expect. Light falls off sharply with distance, so a fixture that is adequate at the top of a tank can be well below the useful threshold at the substrate. In a twenty-four inch tall tank that difference is large, and it explains your specific symptom precisely.
Leggy then rotting at the base is the classic under-lit progression. The plant stretches toward the light, spends its reserves doing it, thins out, and then the constantly damp base of a weakened plant rots. It is not a rot problem with a lighting coincidence; the rot is the last step of the lighting problem.