I never use tank water to thaw frozen food.
but,, that is because I feed several tanks.
I use a small tupperware bowl with lid and float the bowl in a tank.
the water slowly melts the food.
i can they use utensils like spoons to take the food and deliver it tothe tank.
I try to avoid cross contamination using this method. I wash the bowl out with tap water and keep it in the freezer or fridge till the next use.