The best description of how molecules released by fish become nutrients for plants is that fish release ammonia as a waste product through respiration and excretion, and then beneficial bacteria convert this ammonia into nitrates through a process called nitrification. These nitrates are essential nutrients that plants absorb through their roots to make proteins and nucleic acids necessary for growth

. More specifically:

• Fish produce ammonia as a metabolic waste.
• Nitrifying bacteria such as Nitrosomonas convert ammonia into nitrites.
• Another group of bacteria, such as Nitrobacter, convert nitrites into nitrates.
• Plants take up these nitrates as a usable form of nitrogen to support their growth and development.

This process is a key part of nutrient cycling in aquatic ecosystems and aquaponic systems, where fish waste is effectively transformed into plant nutrients by microbial activity