anon, I'm sorry I never got back to this post! It sounds like you've been through some tough experiences with your new tank! We really should have helped you through. Feel free to email me when you have a specific question that needs a timely answer. I can't always get on this board so feel free to email me anytime - firstname.lastname@example.org
Anyway, it sounds like you've learned a great deal in the past month, and while it is sad to have lost so many fish in the process, the experience you gained is invaluable towards keeping your future fish happy and healthy.
I made the very same mistake YEARS ago when I first started the hobby (circa late 1980's). I cleaned out my Whisper filter with bleach, dechlorinated and lost a ton of fish. It was a two-cartridge filter (large model) and subsequently, I made sure to change only ONE filter pad at a time, allowing the old pad to colonize the new with nitrobacter