I recommend a topoff system, to keep that salinity more even: I got one from autotopoff. com, the deluxe kit, which is under a hundred, and runs from a bucket with pump to deliver fresh water to a pre-set fill line 24/7. I have an old salt bucket with a pump and hose, and a hole in the hose to break the siphon set up by the pump. That's one.
A skimmer is really a good thing, too. I use an Urchin, which is pretty economical as skimmers go: fits in the sump, very small footprint. A sump can be as simple as a Rubbermaid tub with a water entry, a barrier of some sort, be it only a plastic box to quiet the flow, and a return pump.
With those three items, you're a lot more automated and your tank will be less prone to spikes and fluctuations in conditions.