How do you keep you salinity at a certain level?

[Coleus]

+1 to refractometer and ato. Also when you do water change, make sure the new fresh water has same salinity with the current tank and make sure you fill exact amount that you withdraw.

Tunze ato is good but if you spend a little more, may you can get apex with io breakout box then you are set. Seriously, once you have a controller, it is so easy to take care of the tank. I would never a saltwater tank running without a controller.

I found with my float switches controlled by Apex controller, i can pump fresh water or new water to my tank with the exact amount.