A pint of water doesn't actually weigh a pound. It's a phrase that gives an approximation of weight, not a scientific measurement.
A US pint of water weighs approximately 1.042 pounds. Which takes a gallon of water to around 8.34 pounds rounded.
Going through the gallon -> liters -> kilos -> pounds conversion, you end up with 8.35 rounded.
To make things more complicated, the weight of water is not consistent. It weighs less at higher temperatures.
See chart here.
For practical purposes, if your scale comes in around 8.34 at room temperature it's probably accurate enough to weigh your fish on and not get misled.
This of course does not account for the container itself. If you need that level of accuracy, weigh the gallon, empty it, and weigh the bottle the next day after the remaining water evaporates. You probably can't do that at the grocery store