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Just thought I would share my experiences of salinity measurement with you. When I first set up my tank this time around (4 months ago) I purchased an Aquarium systems "SeaTest" SG meter for approx. £11.00. This was upon the recommendation of my LFS who told me it was "very good" and accurate. After much reading I chose to run my tank at a SG of 1.025 and have been quite happily maintaining it at that using the "SeaTest". However having read
several posts on various boards including this one about the inaccuracy of these "swing arm" devices, I borrowed a "floating devices" and got a very similar reading with that.

I still wasn't happy and decided to see what else there was. I looked at refractometers and they seemed expensive and slightly messy to use. I then looked at Salinity Monitors that measure %salt by electronic conductivity, and found the "Pinpoint" Salinity monitor for £93.45 available by Mail Order from Fish Antics in Dublin. The monitor arrived today and I calibrated it using the provided 53 mS solution. I then measured the tank salinity and found it to be 44.1 mS (1.0210 SG). I am more lightly to believe this than the other devices as it is using a calibrated machine that is also temperature compensated.

To be certain I tried re-calibrating the monitor but it was fine and measuring my tank resulted in the same reading. OK so no harm done and the occupants have not shown any distress. Whilst not too concerned as to absolute accuracy of any of my tank measurements to the nearest 1% or whatever (as I think trend and changes from the "norm" are more important
generally), I do find this discrepancy totally unacceptable, particularly if I am operating at one of the extremes or need to treat some fish in a quarantine tank at a particular low SG.

It's important to remember that there's a difference between specific gravity and salinity. Specific gravity is really only a measure of the total dissolved stuff in the water. It includes salt, minerals, elements, and anything else. Salinity, on the other hand, is specifically a measure of the salt content of the water. Because of this, the hydrometers you mentioned would always give a higher reading than a dedicated electronic salinity monitor. If your electronic monitor measures specific gravity, it's probably more accurate anyway.

I went through a very similar thing except I lost a whole tank and a 9-year-old favourite from another. A long story, but basically I bought two of those SeaTests and started mixing my water using them. The first water change with them was a big one. The fishes started getting sick and I tested the water for everything. When I couldn't find anything wrong, I did more water changes for good measure. As I had just bought not one, but two of these testers, they were the last things I suspected and by then, it was too late for one tank. My fish were swimming around in fresh water. I was major pi$$ed.

Those SeaTests are highly inaccurate and should be avoided. I am also now using the Pinpoint and have had no problems since. A tough lesson.

Hi Jake! I've got the same style device and find that it works fine. I also invert the thing to take samples from mid column. Sometimes I can't see a bubble (?) on the indicator arrow but I always tap the case a couple of times to knock off any air drops anyway. Also temp will make a difference as I have noticed while preparing new water before it is heated. It is also critical that you hold the thing perfectly level. Let me know if any of this seems to help.

I have an "Aquarium Systems" specific gravity tester, and I tested the salt 10 times in a row, and it gave me a different reading every time. No bubbles were on the meter, and I filled it to the proper level every time. In addition I put the tester in upside down forming a bubble inside it and then turning it right side up letting the bubbles out in order to get the water from the middle of the tank. This is something I made up, just thought it was a good idea. I'm very new at this stuff; any help would be great - not sure why it gave me a different reading every time.