Cheap probes for common ions and parameters?
 

I would like to monitor as many parameters as possible electronically rather than relying on colorimetric assays. Hanna has a number of handheld checkers that test phosphate, calcium, alkalinity, nitrite, etc. Each unit is about $50, surely each of these is just a relatively simple probe with some basic signal processing and an LCD.

Has anyone been able to locate these probes OEM or Chinese knockoffs of them? I tried Alibaba and eBay but had little luck. I'm sure if the Chinese made these probes alone they could be had for $10-$15 each.

EDIT: Just realizing that the Hanna checkers are actually colorimeters...

Does anyone have experience with UV-Vis spectroscopic methods for detecting compounds of interest? They're probably not optically active.

Well....unfortunately, colorimetric methods typically employ a specific reaction that will change a chromophore's optical properties. The alternative of an absorbable measurement (using a molar extinction coefficient) typically requires a relatively pure sample. So, ion selective probes are the best bet, but they tend to drift over time and need to be recalibrated on a regular basis.

Yeah, now that I've had my tank running for a while I've come to realize that the parameters I would really want to measure are temperature, salinity, carbonate hardness, and maybe nitrates.

Unfortunately determining carbonate hardness is difficult though it is possible. Nitrate can probably be measured by ISE.