in tank hydrometer?

[The endler guy]

So I was thinking about hydrometers and would it be possible to cut the face with all the numbers and floaty army thingy (very scientific!) and put that in the aquarium similar to having a thermometer in the aquarium? Idk could be interesting if any one wants to do it be sure to post it!

[gardenman]

I still have a glass Nektonics hydrometer that I just kept floating in my marine tanks all the time. That's "old school" stuff these days as people have moved more to refractometers instead, but it worked for me. You can still buy glass hydrometers. A quick Google search brought up all kinds. Just be sure the one you buy comes in the right range for the water you'll be keeping it in. 

[Chris]

You could totally do that, as long as you don't allow any salt buildup on moving parts.

Refractometers are great for reef tanks where accuracy really matters - most brackish fish (and marine fish, for that matter) just need it close enough, and hydrometers are great for that.