Hello; I added this link a while back and will add it again.
http://www.americanaquariumproducts.com/nitrogen_cycle.html
I use it as a reference from time to time. I wish to direct your attention to two sections of the article in particular. The first is titled; Cycling Summary. The first sentence states that..." water changes are helpful during cycling". The author goes on to give fairly detailed explaination throughout the article.
The next portion I will suggest to read is titled; "WHAT TO DO FOR HIGH AMMONIA OR NITRITES (when fish are already present):" More specifically down a few paragraphs to the "Be aware" paragraph just after Aquarium Cleaning; methods (bowls). I am not confident this is what happened to your big tank, but it fits some of the data you have posted. You mention a few times that things went bad after a large water change. You also mention that you feed heavy which likely also meant there was considerable organic matter undergoing decay. Your tanks had a heavy stocking. You added two fairly large fish shortly before the large water change, for which the bb population was not likely to have had time to increase sufficently to take up the extra ammonia. Again I do not know if this is the problem. It may well be that you had a cascade of issues.
In the end, as least so far, we each get to run our tanks any way we want. Correct me if I am mistaken, but it seems that you want to have large fish and that you feed heavy so to have them grow big quickly. You also tend to stock heavy. To me this leads to what I call a "tightrope tank". In the first decades of my fish keeping I ran a few such tanks. Lots of issues and always just a power outage or some other minor thing away from serious problems.
Again good luck
http://www.americanaquariumproducts.com/nitrogen_cycle.html
I use it as a reference from time to time. I wish to direct your attention to two sections of the article in particular. The first is titled; Cycling Summary. The first sentence states that..." water changes are helpful during cycling". The author goes on to give fairly detailed explaination throughout the article.
The next portion I will suggest to read is titled; "WHAT TO DO FOR HIGH AMMONIA OR NITRITES (when fish are already present):" More specifically down a few paragraphs to the "Be aware" paragraph just after Aquarium Cleaning; methods (bowls). I am not confident this is what happened to your big tank, but it fits some of the data you have posted. You mention a few times that things went bad after a large water change. You also mention that you feed heavy which likely also meant there was considerable organic matter undergoing decay. Your tanks had a heavy stocking. You added two fairly large fish shortly before the large water change, for which the bb population was not likely to have had time to increase sufficently to take up the extra ammonia. Again I do not know if this is the problem. It may well be that you had a cascade of issues.
In the end, as least so far, we each get to run our tanks any way we want. Correct me if I am mistaken, but it seems that you want to have large fish and that you feed heavy so to have them grow big quickly. You also tend to stock heavy. To me this leads to what I call a "tightrope tank". In the first decades of my fish keeping I ran a few such tanks. Lots of issues and always just a power outage or some other minor thing away from serious problems.
Again good luck