I am lad your tank is better, but one thing to clear up about plants.

I do riverwatch and we had to learn all about DO levels and what is normal diffrent times of the day and why.

Plants durring the day, in the sun have photosynthesie, which takes sunlight CO2 and some water. The main byproducts are H ions and O2. Ovously fish are animals and breath O2. At night there is no sunlight for photosynthesis and plants do not use the 02 or the CO2.

What you are thinking of is that when plants die the microbacteria that decomposes them uses 02 to survive and puts CO2 in the water.

SO when your plants die take them out right away.