backflow -- or is my tap water safe to drink?

While I was filling tanks last week, using a direct faucet connection, not a Python connection, I left the end of the hose in the tank for a few minutes after filling the tank. Apparently, about 1/2 inch of tank water back-siphoned into the sink faucet (the tank water level is above the sink slightly)

I have decided to include a back-flow preventer from the hardware store, drip watering department, in my direct connection. I don't know if that can happen when using a Python if you leave it on "fill" and turn off the water, then leave it in the tank.

My son said the water from the refrigerator door dispenser tasted fishy to him, guess I need to change that water filter.

I just hope I can assume that the chlorine in the lines has killed anything that might want to grow in my water system.

Not thinking well...

I was not thinking it through, you are right about the pressure.

I was following some cautions related to drip watering system and sprinkler systems and outdoor ponds. The deal there is that you need a backflow preventer to be certain that no outside water gets into the system. This could happen either during floods, when the water pressure could be higher than the line pressure, or anytime when linepressure is down during city emergencies, or even if the city decides to flush out the lines or do repairs and turns off the water... then the small amount of pressure of any verticeal pipes or full hoses would be greater than the line pressure, and the line pressure could actually turn to suction for a moment.

All of this is insignificant during my water changes.

The dropping level must have been from leaks in the hose connections at the sink.

False alarm, my error (smacking head)