Is it possible for a Python siphon to back-flow into drinking water supply?

I just had some workers from my city come and test my tap water over some water quality concerns of mine. I explained that I'm an aquarium hobbyist and I test my water regularly, along with the tap water. They asked if I used tap water to fill my aquariums and I said yes and explained how I use the python siphon to do water changes. They said the python siphon is dangerous because there's "no true back-flow stop" and that while I'm siphoning water out of my aquarium, it could possibly back-flow into my faucet and down into the other systems of my water and other people's water. They said I could possibly be putting dirty aquarium water into other people's drinking water (I live in an apartment complex). This scared me a little.

...until I started thinking about how the python works. I'm wondering if they just didn't understand how it operates. When I'm siphoning water out of the aquarium, the faucet is on full blast, and the adapter is in the open position. The pressure sucks the water out of the aquarium, through the tube, and into the sink (anyone who uses a python or something similar knows what I'm talking about). I don't know how any aquarium water would be able to get into and down my faucet with the tap water being shot out at full blast. Does anyone think they know if the workers have any real basis in this claim, or did they just not understand the operation of the siphon? I obviously don't want to be putting dirty aquarium water into everyone's drinking water supply if that's what indeed is happening.

 

Well, outdoor faucets are required to have anti-siphon valves so I guess it's possible...

https://www.hunker.com/12292574/why-do-anti-siphon-valves-leak-water

 

Unless you lose all water pressure in your tap it wont back flow. Secondly at your meter by law there needs to be a back flow resister so it is impossible to contaminate the town. Worst case some might get into your faucet.

 

Common sense tells me that this can't happen. Water doesn't flow uphill. If the tank water is mixing with the water coming out of the faucet all of the water will be going down the drain not back up into the faucet.

 

Only way that could happen is if there was a major break in your plumbing pipes and or from the street, then yes it could happen.