More information will help, as there could be multiple issues interacting.
Can you post tap water data, namely hardness, pH, ammonia, nitrite and nitrate. Hardness you can ascertain from the water supply folks, and pH to confirm your own test. Ammonia, nitrite and nitrate may be present in the water, this is essential to know. Also, any toxins in the water according to the data from the water folks? Higfh levels of this or that metal can be problematic--safe for humans is often not safe for fish.
Also, what is the pH in the aquarium? And what is the ammonia number? And nitrite and nitrate too please.
The temp at 84F is too hot for those fish, and this makes matters worse because higher temp means less available oxygen so it further stresses the fish, plus the ammonia. Probably also a contributor of plant demise which will further lessen water quality.
The pleco and CAE may be problems, contributing or not now but they will later. What type of pleco and how large is it now?