Beware of the RedSea Hydrometer with the little built in thermometer. It costs about $14-15 at LFSs and maybe slightly less at the major chains.
I've had this hydrometer since I started my tank, coming up on 3 months ago. I've been trying to use it to keep my salinity near 1.025. At first I kept the water where it read 1.025. Then I took some water to the LFS to double verify all of my parameters and they said my SG was pretty high. After doing some research, I found that some hydrometers are calibratedt o be accurate at 60°F water temps and that at 80° they will read lower than they really are. The conversion seems to be about .0025 at 20° warmer water so since then, I've been keepign my SG between 1.022-1.023 on my RedSea hydrometer with the little thermometer. I went to buy fish last night and took a water sample to be double verified that their test came up the same as mine. I also took my hydrometer to check it out vs theirs. They tested my water and the SG came back as 1.028 as opposed to the 1.0225 that my hydrometer read. Then we verified again using their tank water and it read 1.016 vs their 1.020 reading. I stopped by Petco on the way home and they tested my water and came up with the same 1.028. So it turns out I've been keeping my salinity through the roof. It now makes me wonder if that could be what killed the two clowns I had, if the salinity is that high then the salt displaces oxygen possibly causing my fish to suffocate? My inverts have all been doing great.
Anyways, moral of the story, double verify the water readings you perform at home at least several times per year to make sure your testing equipment is functioning properly. And you might avoid buying the RedSea hydrometer with the little thermometer built in.
on a good note, I've been using a RedSea marine test kit for my water testing. Since buying it several months ago, I've read several post suggesting staying away from RedSea water tests. Just want to point out that the LFS came up with similar readings that I saw with my test kit, which was
Am - 0
Nitrite - 0
Nitrate - 0
pH - 8.4 (my test read ~8.3)
Alk - 10.9 (my test read ~3.5meq/l, which calc to about 10.5)
so in summation, RedSea water test kit... decent. RedSea hydormeter with built in thermometer... bad. Unless I just got a bum unit.
Double check your readings several times per year with different equiptment.