Hmmm...
No light = plants not photosynthesising, not producing oxygen = less oxygen, possibly not enough.
Did you get an airpump and aerate the tank?
Dead fish left in-tank for unknown periods, possibly hours = ammonia spike = ammonia poisoning = more dead fish = vicious cycle.
Did you stop CO2? If not, and the plants were not photosynthesising, the CO2 levels could have gone up to toxic levels and also possibly cause a pH crash.
Anyway, what was the logic for turning off the lights?
Stealthy fish = stressed fish.
Solution:
Large water change, but at most 50%. Get the plants photosynthesising again... i.e. CO2 + lights + fetilisers. Check product literature and internet whether the medication affects plants.
Continue the salt treament plus temp adjustment, that usually works by itself without additional medication. Replenish the salt lost from the water change.
Everything's going to be fine. For a planted tank, I try not to over do treatment for the fish. I find it's better to keep the plants healthy and they in turn help take care of the fish.
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