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Hydrometers are notorious for being unreliable. If you insist on using one, make sure you rinse it with FW
after each use or better yet, rinse it with Ro
water and let it sit filled over night before dumping it. Then, take it to the LFS
and have it lested against a refractometer so you know just how accurate it really is. When using it, fill it to the line and then make sure you tap it against a level surface several times to remove any air bubbles as these can effect the reading.
For best results, I recommend you invest in a refractometer so you get much more accurate readings. You can find them for pretty inexpensive on several online sites. It is a tool you should seriously consider.