Hi trem1 Welcome to Fish Lore
I'm sorry to hear that your fishies are dying. Perhaps it's not your cleaning routine, but something else going on?
Here's my routine:
1. Test the water for Ammonia, Nitrites and Nitrates...write them in a note book.
2. Unplug filter and heater.
(Depending on how much water you take out this may not be necessary)
3. Using a siphon/gravel vacuum, I vacuum the gravel, the water goes into a bucket that's only used for this purpose.
4. Swish filter media, decorations and plastic/silk plants (if needed) in used tank water, return to tank.
5. Fill another bucket (again, only used for this purpose) with water approximately the same temperature that I took out.
6. Treat the new water with a conditioner.
7. Slowly add clean treated water back into the tank.
8. Fill filter with water and plug that and the heater back in.
I do this once a week.
Hope this helps. Good luck.