Canisters are known Nitrate factories. They have to be serviced frequently for a saltwater tank as they only offer mechanical (little chemical) filtration. API test kits are less than accurate in many cases, and your temperature is about 3-4 degrees too warm supporting any bacterial blooms.
Bring salt down SLOWLY to 1.025 and assure there is adequate oxygen in tank.
AGE OF TANK ??
I would recommend with use of API kits to grab a good water sample and take to a trusted LFS and have them test for you to see what readings they come up with and to compare to yours.