i just noticed my saltwater fish tank didn't have as much water as it did before.so i added some freshwater and checked the sg it was still the same!!can someone tell me how come it is still the same sg?

What's sg? If it's the salt level then that's because salt doesn't evaporate so unless you physically take it out and replace with fresh, the salt level should remain the same.

I note you said you added freshwater to your tank.I'm assuming you didnt mix it with sea salt.Whats your salinity/specific gravity exactly as for saltwater you want salinity/specific gravity to be 1.020 to 1.023 at 75F.If your salinity/specific gravity is within the correct range I wouldn't worry about it.

Jen SG is specific gravity.You use a hydrometer to get reading for it.

Ahhhhh - runs off to look up specific gravity

it is1.o20

Salt doesn't evaporate so when you are replacing evaporated water you always use fresh water...preferably RO water....When you do water changes and actually remove water then you replace it with salt mixed water....