hmmm......did this tank come this way and was setup somewhere else and ran for xx months and you have only had it at your house 3.5 weeks? or was it just setup that fast?
Have you checked the expiry on your test kits?
Are you running any kind of chemical or mechanical media? (like carbon, filter floss, etc.)
I keep my salinity at 1.025---->I agree with Michika (missed that!)
Maybe you could get some kind of nirate sponge to soak it up (nitrate) until you find the source of what is causing it. It is possible it could be overfeeding, could be nitrate/phos stuck in the LR itself, could be in the sandbed.
On the other hand it could be you have an invert predator you don't know about? Perhaps a trapdoor snail (found one of those in my tank last night trying to eat my snail! >
or a hidden crab could be the culprit....