If it's the plastic style hydrometer that you scoop water into, then they can be pretty easily thrown out of wack. There's a little plastic washer that's part of the needle. If that gets gummed up it will give you false readings. I never trust those things. Use a floating hydrometer if you're not going to get yourself a refractometer
I keep my tank at 1.025 But that's a reef tank. I've kept FOWLERs between 1.022-1.025