The water is just tap water from the faucet. The tap water where I live is clean though, compared to my college's city water which I think has hard water.
So whenever I or my parents change my turtle's water, we first take the turtles and put them in a plastic tub (where they get fed). We remove the dock, and then carry the glass tank and pour the water in the sink (and clean the sink afterwards). We've been carrying/using the same 15 gallon glass tank for at least 10 years...I'm sure this isn't the correct method. Over these years, my turtles have grown a lot and the 15 gallon tank seems so small and it makes me sad to see them have not a lot of space.
Sometimes the water can get dirty quickly, so we have to change it earlier than a week. But we typically change the water 1x a week. Could they get sick from their own water, like if the water has their poop?
Unfortunately, I don't even know the gender of my turtles
Their nails are short, which I'm assuming they are females but I don't know how to correctly tell.