[Sea Witch (OP)]

I have a question about how to test for certain elements and compounds in water.

I keep aquariums, both freshwater and saltwater.  The things I need to (or would like to) test for on a regular basis include the following:
Ammonia in ppm
Nitrite in ppm
Nitrate in ppm
Phosphate
Alkalinity in meq/L or ppm CaCO₃ equivalent
Salinity and/or Specific Gravity
Magnesium
Dissolved Oxygen

and there are others, either easy to test for or less important.  The majority of "test kits" readily available to the hobby folks are poor quality.  Can you tell me what can I buy that would be more precise and accurate and reliable than what is available in pet stores?  What do I ask for and where could I find it?  I don't need the best tools known to man, but there's got to be something better if one knows what to look for.  Thank you.

[Mazurka]

Also being an aquarist, (although I have never had a tropical marine environment) I know exactly what you mean.  Most of the petshop testkits give pretty vague and unsatisfactory results.

I looked into the availability of suitable digital instruments / handheld meters and as far as I recall, came to the conclusion that there was no single instrument capable of measuring the determinands required for fish keeping.  These units were also quite expensive.

I also looked into the sending samples away to an accredited laboratory – and was quoted prices of > £75 for non commercial numbers of samples.

I did not look at all of the glassware needed to titrate my way to an answer (due to not having the room or a sufficiently long life to justify it)

When I kept a coldwater marine tank, I relied on a hydrometer to keep an eye on salinity.  Simple and accurate.

Rather than inconclusive testing, I now rely on a weekly water change (10%) and a 25% change every other month (along with a check on the filter).  It seems that most fishy ailments are down to poor water and unless there is a real problem that won’t go away with water changes (which probably will not show up with petshop test kits) I would not bother.  (I understand from a chum that if you have corals and need to check the Mg / Ca concentration the testkits are “accurate enough”)

[Bored chemist]

The problem is that the test kits are not designed to be all that good because the makers know that if you need accurate results you will analyse the stuff properly. Sending it to a lab might not help much because the water will change in the post.

If I was going to set up to measure those at home I'd look at methods for titrating most of them. An ion selective electrode system would be good too, but rather expensive.