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Old 02-01-2012, 11:21 AM   #1
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Measuring Salinity for Brackish Water

I am working on a build for a fiddler crab tank, and I can't seem to get the salinity right. I feel like my hydrometer (which is the kind with the moving dial) might not be working properly. Let me give you an idea of what is going on.

The salinity should be around 1.008 for fiddler crabs. The tank is a 30 gal, but less than half is filled with water (I'm doing half land, half water for the crabs). I'm guessing it has about 10 gals of water in it. When I first put water in I put a little less than half a cup. It's hard to determine how much since the instructions on the salt are for salinity of 1.022. After waiting a few hours I tested the salinity, which read at 1.013. I did not do anything, but waited until the next day to re-test. The next day the salinity read 1.004. I don't really understand how it could change so much in one day... But since I'm new to this I added some salt (probably about 1/4 of a cup). The next day I tested it and it read 1.002. So I feel like something is going on with my hydrometer. When testing my tap water sometimes it says it has salt, sometimes it doesn't. I know that there are bubbles in it at times, and that messes up the reading.

So basically I need a better understanding of how these things work, or if I should take it back and try purchasing another. I don't think the hydrometers that just float in the water would work, since the water is not very deep. I'm also not spending 50+ bucks on something else to test the salinity, because I'm not made of money. Suggestions?
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Old 02-01-2012, 12:00 PM   #2
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This is why you should wait 24 hours after mixing the water to test it. Try buying the glass hydrometer they are cheap and you can compare the results.
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Old 02-01-2012, 12:07 PM   #3
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The plastic hydrometers with the swing arms are notoriously innaccurate. When using them, you must be absolutely sure that any and all air bubbles have been removed as they cause the hydrometer to be off. Generally, its accepted that if you need an accurate salinity measurement you need to use a refractometer. I think you can pick up a pretty good one for around $30 when they're on sale. Of course people kept tanks without them for many years. I've personally had much better luck with the glass ones mentioned above then with the plastic swing arm variety. Strive for consistency.
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Old 02-01-2012, 12:12 PM   #4
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The only glass one I saw was very long, and my water is shallow. That's why I'm not sure if they will work. Are there smaller versions out there?
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Old 02-01-2012, 12:16 PM   #5
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Yes, but you will still have the same issue, as they have to float. Mix water in a bucket prior to placing in your tank. Once you know the water level, keeping it at that level should maintain the same salinity.
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Old 02-01-2012, 12:21 PM   #6
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Ok, I'll see what I can do.
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Old 02-01-2012, 06:27 PM   #7
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I have a long pilsner glass that i use to measure it in. Its long enough that it doesnt touch the bottom
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Old 02-01-2012, 10:11 PM   #8
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Refractometer will make your life a lot easier and offer you peace of mind
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Old 02-02-2012, 04:07 AM   #9
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Refractometer will make your life a lot easier and offer you peace of mind
i have done pretty well so far and my fish seem very happy and im sure it will make it easier but if i dont need to spend the money right now id rather not. as long as my fish are happy i happy.
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Old 02-02-2012, 09:35 AM   #10
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Originally Posted by BrackAttack View Post
I have a long pilsner glass that i use to measure it in. Its long enough that it doesnt touch the bottom
That's a good idea. Thanks.
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Old 02-03-2012, 05:11 PM   #11
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Ok, we added salt yesterday, waited 24 hours, and tested today. It read 1.005. We are only adding like 1/4 cup at a time, so I could understand the small increase. I'm thinking since the LFS I'm buying the crabs at has them in freshwater I will stay with the 1.005 for now. Then once I get them I will gradually increase it, and buy a better hydrometer!
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Old 02-03-2012, 06:16 PM   #12
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Do they need to be in a higher sg then that?
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Old 02-03-2012, 08:18 PM   #13
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They need 1.008.
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