I hear you. Sometimes I make water and don't get around to the actual change for a few days, so there is evap. Just add water back to the line on the inside of the bucket. I you do it the same every time there is no reason why it would be different.

The most accurate way i have found is to dip my finger in the water and taste it, if it doesnt taste very salty i just grab the mortons and dump it in. seems to be working great, the three tangs in my 12gallon nanocube are so happy they jump for joy.(somtimes ending in their death) but hey its their life who im i to judge. seriously though, my LFS is cool and will test it for free, i have a hydro and its way low.

Depends on the units chosen. Floating hydrometers can be very accurate - if used properly. For speed and consistency though, the refractometer is the winner. They are virtually idiot-proof.

The swing arm dealies, some may be accurate enough for a while, but over time they tend to become less and less accurate and precise. If your livestock choices favor things that can handle some changes in salinity this may never appear to be a problem to you. If you are really looking for consistency though, outside of some spendy lab equipment, a refractometer is the way to go.

refractometer. came with the tank i got for free from my sister-in-law who couldn't keep up with its needs.

refractometer

this was a question from 2005...

I use the floating hydro. Toss it in the tank before a WC and toss it in the bucket before I add.

Side note, I use the floating hydro to measure specific gravity on my home brew as well. Not the same one for both. That'd be like ghost busters crossing streams.

I just watched that yesterday on another side note.
Wish I could just smell it and say yeah this is about the ocean.

But I use one of those floating hydrometer thermometer combos, although in the bucket I cant get my face low enough to read it so I have to put it in a tall bottle first, slightly annoying.

I occasionally use my refractometer but always find myself grabbing for the hydrometer. Its actually pretty darn close in it's readings compared to the refractometer.

refractometer.

Sybon Refractometer calibrated with 53.0 mS (35ppt) solution from PinPoint.

Floating Hydrometer. Seems to be accurate so far as I can tell, reads dead zero in RO water, 1.025 in my tank.