Anything out of balance can make them act [i][different/i]. A sick fish will some times be picked on, thus inviting aggression from the others.
I have white clouds in my cool water tank, they prefer water a little cooler than tropical but is not going to harm them, they chase and nip at each other all day long but no damage is done.
Ammonia converts to Nitrites, Nitrites converts to Nitrates. Nitrate is less lethal than Ammonia or Nitrite. But if it gets too high it can kill your fish in a very short while. If you have lots of live plants they will absorb a good bit of the Nitrates. Other than to get pleco poo out

to keep the Nitrates down is one of the best reasons to do water changes. In my opinion the large filter is a plus it will keep most particulate matter from floating around your tank and make it look sparkling clear.
Oh and welcome to FishLore!!!!
Mags I think you have the prep down pat. Dechlor, close to the same temp. Then change water.
Ok I'll end this book

Yell if you need anything else.
Carol