Final answer:
To help decrease the concentration of ammonia in the aquarium, the researcher should add Nitrosomonas and Nitrobacter bacteria, as well as plants.
Step-by-step explanation:
The researcher should add Nitrosomonas and Nitrobacter bacteria to the aquarium to help decrease the concentration of ammonia. Nitrosomonas bacteria convert ammonia into nitrites, while Nitrobacter bacteria further convert the nitrites into nitrates. This process, known as nitrification, helps to reduce the concentration of ammonia in the aquarium.
In addition to the bacteria, the researcher could also add plants to the aquarium. Plants can help decrease the concentration of ammonia by absorbing it as a nutrient for growth.