OK, i'd say ntrite is almost definately what caursed the 11 deaths. Your pre-death symptoms are all consitant with poisoning, and your water stats confirm this. I would advise enough 50-75% waterchanges to bring the nitrite down BELOW 0.25. Any level above 0.25, for ammonia and nitrite, will prove faital, often within hours. Nitrate above 50ppm will caurse long-tearm damage, so ensure this does not raise above this value. Once the cycle is complete, you want to start doing 25% water changes each week, and work from there depending uppon your nitrate values. Untill the cycle completes, you need to run water tests twice dayly, water changing as nessisary. On waterchanges, you only need to add tap water conditioner OR declorinator. The other products you mentioned are generaly considered to be snake-skin oil. I have read elsewhere on the forum, that salt may caurse damage if used for prolonged periods of time, as it puts the internal organs of your fish under a great deal of additional load. The only exception is the mollies, which are technicaly brackish, but they adjust to life without salt better then the other fish adjust to it.
HTH clear things up
Rabbut