When having problems it's best to eliminate all known sources of difficulties.
1. water: too many to name, from anti-chloramine additives to chloramine, arsenic, etc. Ro/di is safest. Try that, if what you're doing isn't working. What goes into water that hurts fish isn't necessarily just the chloramine: it's polyviny-whatever-contaminants and arsenic and other things the government thinks is safe enough for us to drink in tap water---it will, however, prove lethal for some sensitive fish. The anti-chloramine products don't remove those things. Sometimes water comes in high in nitrates, from farm runoff (ours has gone up a factor of 12 in the past 10 years.) The water-adjuster doesn't touch that. Ro/di filtration does.
2. don't get any more fish until you have your own test kits and your system is stable.
3. mechanical filtration (cannisters/filter pads) are actually far trickier than the sump/rock/sandbed method. If overloaded with muck, it starts building up lethal levels of various things, and it has a way of delivering it all of a sudden, sending levels sky high and killing your most vulnerable (and usually favorite) fish. If you use that method instead of a sump be absolutely scrupulous about a cleaning schedule, and stick to it. Running a sump/topoff/skimmer has its schedule, too, and a rigorous one, or it will collapse---it's just a bit steadier, and because of the requisite testing, gives off more alarms. It's a case of pick your method and do it well.
Hope that helps.
