A liter is a little more than a quart (about 1.06 quarts), so I'm guessing that 10 liters would be roughly 2 and 3/4 gallons (?). This is very possible, particularly when you have a large surface area for water to evaporate from and in the winter when the air is much dryer. I'm always adding water to the stock tanks.
As for partial water changes, I think they need to be done more frequently than once a month (although some of the water that will be replaced due to evaporation). (You should get some testing kits and check for ammonia, nitrites and nitrates). A high nitrate level can be lowered with a partial water change.
Yes, canister filters can be wonderful.

"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-