Can too much light kill fish?

I have a 20 L aquarium, mechanical filter, lights with timer, everything was fine. Since the (live) plants were not "too" alive, I upgraded the light. Instead of the generic LED lamp installed originally, I installed a ribbon of generic LEDs of "slightly" higher power.

The obvious change: the plants which were barely alive, barely multiplying, have gone wild growing and multiplying. Of course, not the best thing for the fish - since they had a little less room for swimming. But still, everything fine.

Until, suddenly, during 1 week, all mollies died. Several adults, and several generations of babies. Since babies died too, I can safely rule out old age as the reason of death. Some bottom dweller died too, but in its case, it might have been age (it was at least 2 years old, I guess). The shrimps (cherry) and the 2 snails seem unaffected.

I tested the water with an NO3- test kit. The color of the solution did not even try to get pink (it remained yellowish) - so I guess that is "perfection"?

So the bottom line, why did all the fish die suddenly?

Notes:

1. A light dimmer is planned to be added to the lighting system.
2. A lot of the plants will be removed from the aquarium - I will have to find a new home for them.
3. The plants are: Java moss and Vallisneria. When it was small, Vallisneria looked like "Corkscrew", but now (big) it is not really spiraled anymore.

My assumption was that higher light -> happier plants -> less NOx -> happier fish. But something in this thinking must be wrong.

I hesitate to buy new fish, before I understand what is going on. And an aquarium without happy (living) fish is not very appealing to me.

Update 1:

1. I intended to write about it initially, but I forgot. Thanks to the people commenting for reminding me: I do not see any relevant algae problems in the aquarium. There are a few green spots on the inside of the glass, but they did not extend in a very long time. The plants seem clean also.
2. I noticed recently (pretty much while the fish were dying) that there was some foam forming at the surface of the water - while NOT having an adult betta splendens. The foam (with large, loose bubbles) took a very small area (about half of a banking card), but it was there clearly visible. I could not find any explanation about it.
3. I promptly removed all dead fish as soon as I noticed them, to avoid escalating problems.
4. For about a week (maybe slightly more), the mechanical filter did not work properly - it trapped air inside, and the water flow was limited - also limiting the ability to aerate the water. Clearing the sponge did not help. Tricks to fill the filter with water did not have a lasting effect (somehow, the filter emptied some of the water inside in favor of air - obvious by listening to the specific sound inside the filter). The problem was finally solved when I disassembled the filter 100% (all components spread on the table one by one), and cleaned them of all accumulated debris (no chemicals, just clean water).
5. I do not even have a thermometer for the aquarium :D I will have to get one, I guess.
6. The timer turns the lights ON at 8 a.m. and turns them OFF at 10 p.m. (so a total of 14 hours of light per day).