Neons are fairly fragile, no doubt the stress alone made them susceptible. Since you didn't mention dechlorination, that's one possibility, another is temperature shock if you didn't match the temperature of the refill water with that already in the tank. The other possibility is that you've had the tank for a number of months and that this is the first cleaning. A 100% water change after a prolonged period, while good from a cleaning out bad chemicals perspective, is very hard on the fish, since it represents a sudden change in water chemistry. A final suspect is that you used the same bucket you use for floor/car/etc. washing and introduced detergents to the tank.
Your pH is fine, an echo in here?
Throw out the "perfect pH", your pH is already perfect, i.e. stable.
For maintenance:
-Feed once per day and only very small amounts. Some flake for the tetras, something sinking for the bottom feeders. What kind of loach, btw. Most loaches get too big for a 10g tank.
-Get a new 5g bucket that will be dedicated to the aquarium.
-50% weekly water change using a gravel vac to clean the substrate while changing the water.
-Every 2 weeks or so (or whenever the water flow starts to slow) rince out your filter media in the old tank water in the bucket. Jinxxx mentioned it, but I wanted to specify that you should rinse it in the old water, not in the tank.
Follow these directions and you should have healthy fish and a healthy tank for years to come.
Thanks Chem for pointing that out.
The fish you have left, doing okay?