Never rely on your equipment as mrrrkva said. I had a hydrometer (swing arm type) that told me my salinity was perfect. One day I decided to take my water to the LFS
because my ammonia kept showing a trace and I was sure it was wrong. Well the ammonia was at 0 as I suspected, but my salinity was at 1.030 even though I was sure it was at 1.025. Just for kicks I spent $10.00 on another swing arm and when I got home and remeasured I was still at 1.030. No animals were hurt, but a lesson was learned.