There's no nitrite, nitrate, or ammonia meters available to hobbyists. You'll have to test with standard chemical test kits. Most pH meters are accurate these days. Pinpoint is probably the most readily available and they work just fine. The Hanna ones are fine too. Salinity meters are not as accurate as an analog refractometer (which are also easier to use). I would recommend you get a refractometer versions an electronic salinity meter.
There are a few ISE (ion selective electrode) meters out there that will work on seawater for nitrate, nitrite and ammonia. But these technology is too expensive for hobby, and not as useful as it sounds. Just keep in mind that quite few pH meter are fine for hobby, but considering that industrial salinity sensor can cost more than a passenger car, what you get for a hobby meter is not that accurate, and quality varies.
