Where has this been my whole life?? Has anyone used or have this? When I saw it at the store, I thought… SHUT UP AND TAKE MY MONEY!! You place it in the tank wherever you like. I have it on the left side of the glass so you don’t see it. All you do is glance over and it shows you where the salinity is. Saves you from having to either put your hands in the tank or go through using the refractor…not that it’s difficult to do or takes a lot of time but you get what I mean. It’s extremely convenient! I did however still check it a second way until I trusted it. I definitely trust this tool. Its been extremely accurate!! It does take a day or less to work. When you first put it in, it will go all the way to the top. Just leave it and wait. The next day is when it started working. Great tool and highly highly recommend! I got mine from Petco for $14.99