Welcome to the forum G&S.
I must say that I am not as encouraged as you seem to be with your present progress. Let me be blunt but please do not take this personally.
A tank that has only been set up for around a month is not yet mature in terms of its ability to deal with ammonia. All fish move water through their gills and in the process absorb a bit of oxygen and remove both ammonia and CO2 from their blood. The CO2 is exchanged at the water surface with O2 and that provides the next "breath" for the fish. The ammonia is retained in the water as ammonium hydroxide in an aquaeous solution until it can either be removed by conversion to nitrites or it can be removed by gas exchange at the water's surface. Until the concentrations become quite high, the exchange at the water surface can be ignored as a thing that will simply not happen.
What we face, as fish keepers, is the need to remove that same ammonia while it is at very low levels. At as little as 0.25 ppm of ammonia, the chemical is approaching damaging levels. The way that we deal with that chemical is to encourage bacteria to develop in our filter that can process the ammonia into nitrite. As it turns out, nitrites are also lethal in fairly low doses. Other bacteria exist that will process nitrites into nitrates, which are comparatively low in toxicity. A mature filter will contain colonies of both ammonia processing bacteria and nitrite processing bacteria. The typical time involved in reaching a viable colony of both types is 2 to 3 months, not one month.
Please do not buy any more fish for now. Right now you are seeing fish dying from chemical poisoning, even though the pH and similar chemicals may read just fine. I am going to disagree with your statement. Your fish did not die in perfect water, they died in water that had some proper chemistry measurements but that had been overstressed in terms of the biological loading. Until you have the chance to develop a proper colony of bacteria in your filter media, no more fish should be added to the tank.
OK so where are we with your tank? In my opinion, you need to cycle your tank before adding any more fish. If fish die during that time, do not replace them. We have some excellent articles by members here for how to go about a fish-in cycle, the situation you are finding yourself in. Please follow the link in my signature area to the article on a fish-in cycle. It will give you some direction on how to go about things and what you should measure along the way to ensure things are going well for you. If you have any questions about your cycle, please return here or better yet to the new fish tank area. We are always happy to help a new person through this trying part of a new hobby that they may not have fully understood when they first got into it.
