Plant decomposition - good or bad?

I would consider plant decomposition to be bad. When plants decompose, they release a lot of nutrients and organic matter into the water. This will fuel bacteria growth (water cloudiness) and oxygen uptake, which could hurt/kill the fish. Keep your water oxygenated to protect fish.

Healthy plants take up nutrients and purify the water for fish. Dying plants release nutrients and contaminate the water.

Thanks Diana - I'm honored!

Dying plants are not only bad but shows that you have a problem with the aquarium.

Decomposition of plant material is nature's way of recycling nutrients and is not bad if it occurs in matured tanks. Leaf litter is used in many tanks to form the necessary environment for the occupants. In over-clean or new tanks which have not yet developed the required bacterial flora to deal with plant litter, you will often see fogginess due to bacterial boom, but that passes away. This recycling of nutrients is need for healthy growth of plants, but most hobbyist take the recourse of keeping their aquariums clean and add madeup macro/micro nutrients.