water changes?

[mark]

When I do water changes I siphon mostly from the display and only about 40l from the basement sump and fuge but do know the total amount I'm taking out.

When I'm ready to start adding the water back, I just turn on the return pump to fill the display back up and start adding the known amount of water back to the sump. If it's looking like the sump wouldn't hold all the water, I'll just shut off the transfer pump, and if it looks like the return pump will start to run dry, I'll just shut it off until the transfer pump catches up.