[jeremyrutman]

I have a reef fish tank (saltwater) I'm setting up and see that there's a number of water parameters people monitor . I'd prefer having an online monitor rather than hand-checking with reagent bottles, so I found I can monitor ammonia in gas and alkalinity in water with  cheap meters; for nitrate/nitrite it seems I'd need a spectrometer.
Are there cheap spectrometers (like 200$ or less) to be had that would let me check nitrate (which has a 200nm-220nm absorption, apparently)?
Alternatively is there another approach to this?
 
Things to monitor in fish tank water:
ammonia 
alkalinity   
nirate
nitrite
Phosphate
calcium
magnesium
ph

[Corribus]

Even a basic model (single wavelength) Visible spectrometer will probably run you a few thousand dollars if you buy it new. UV capability will add a lot of cost because you need an additional light source. You may be able to find an old, used model on the cheap but 200$ or less may be a stiff ask.

Keep in mind that the analysis isn't as simple as it sounds and you won't be able to ditch the reagents completely. If you want a concentration, you'll have to maintain a series of concentration standards. You'll also need to do some method development to ensure there are no interferences.

If you have no background in chemistry, this may be a challenging analysis to get going.

[jeremyrutman]

From this manual here I got some pointers  - it seems I only need a few wavelengths e.g. 220nm and 340nm which I can detect with diodes like this .  The light source can also apparently be a cheap xenon lamp , avoiding the 50$ for a deuterium lamp. I was thinking I cold increase athe pathlength for higher sensitivity but the extinction is already around 99% at 10cm pathlength for the levels I am anticipating (5-10ppm) .
So for nitrate and nitrate it looks like I have a decent plan. I imagine the elements (Mg, Ca)  and phosphate will be tougher but apparently those are less crucial than ammonia and nitrate.
nitrate absorption curves