Can too much light kill fish?

Question

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).

Answer 1

While plants produce oxygene while processing photosythesis during periods of "lights on", plant consume oxygene over periods with "lights off".

If the tank contains enough plants in a comparable "small amount" of water, the oxygen consumtion over night could reduce the oxygene of the water to zero. This would cause the fishes to die. (Maybe bigger ones faster than smaller ones...)

High temperatures can increase this effect, because cold water has a better capability to store oxygene than warm water. But in general LED light does not warm up waters like for example common light bulbs would do.

Answer 2

No. I have seen a salt tank with coral ( and fish ) with 2000 + watts over a 150 gal tank. The big ones were metal halide but he also had other types of lights. There was a salt water club meeting at his house ; it was easy to find as there was bright light coming out the house window. As @ trond hansen points out , refrigeration is required to keep water cool with this level of light.

Answer 3

Hard to tell what went wrong, but nitrate poisoning isn't the only risk.

There's also nitrite poisoning for example (related but separate), as well as various other chemicals that might have gotten into the water.

But the main risk with too many lights, especially for a small tank like that, is temperature. Lamps emit not just light but heat, and a small tank has trouble enough shedding excess heat as is, place bright lights overhead and that can lead to overheating unless you have an active cooling system in place (which most people don't, they're bulky and expensive, and can be noisy).

Even without the heat though, those bright lights may cause unwanted algal growth, algae that can (depending on species) leach toxic chemicals into the water (but you'd probably have noticed it if blue or black algae started growing).