Sorry to hear about your loss with your new aquisitions.
A couple of things could be going on with your tank. A couple of questions for you.
(1) If you vacuum the substrate, are you doing the whole bottom at one time or are you doing half of it per week? If you are doing the entire substrate at one time, that could cause you a problem with your bio-system.
(2) When you change out your filter media, are you replacing everything at one time or do you first change out the sponge or floss and then wait a week and change out the carbon? If you are cleaning the filter and also change out all of the media, that will cause you to have an ammonia spike.
There are a couple of things that I have learned over the last 20+ years of keeping fish, whether it be a reef tank, Amazon tank, or a African Cichlid tank.
If I quit being so anal about the water parameters of the tank and just let it be, everything seems to thrive. Other than my typical waterchanges, I very rarely vacuum the substrate. If I do, it's just to remove the detritus that has accumulated on the top. And I don't mess with the water parameters of my tapwater. I just let everything be what it will be. Consistency is more important than constantly adjusting the parameters to suit what is recommended for the given fish one has in their tank. Unless you are keeping wild caught fish, most of the fish that you will buy at a local fish store have been captive raised for many generations. Their filtration system, such as Pet Smart, is a centralized system for all of their freshwater tanks. They don't worry about exacting parameters. The Amazonian fish will be in water with the same parameters as African Cichlids.
Try not to get discouraged. Ther was a time when I couldn't keep Angel fish and neons alive. I stayed away from them for awhile and eventually gave it another try. Once I did come back to them, I didn't have any problems with them and I still have them to this day.
I had a reef tank setup at one time. Once I had to move it to the basement to make room upstairs. It eventually became an, out of site, out of mind tank. For a year, I rarely messed with it. Not only did everything survive, but it thrived. I had an anemone in there that was about 4 inches in diameter when I put it in the tank and a year later, while it was setup in the basement, it grew to the size of a basketball. I traded it in at That Pet Place in Lancaster, Pa and for the next couple of years of going there, the people in the fishroom knew me because of that Anemone. They said that it was the largest tank raised Anemone that they had ever seen.
