Different kits rely on different technologies to create their results. Some of the methodologies for the kits are such that their interpretation is more difficult than other methodologies.
Most of the dry strip test kits are OK when looking at a "yes" or "no" answer. For example, "Do I have Ammonia?" is either a yes or no answer, as IF you have ANY ammonia, THEN you have a problem, and the knowledge of whether it is low or moderately low or high is generally moot: the mere presence of ammonia is a huge issue. Knowing the exact value of ammonia or nitrite within a
+10% error of margin is unnecessary, we are usually just wanting to know if it is present, and if it is, then whether it is at a low, moderate, or high level. These values usually determine our progress in a cycling tank, or if there are issues on potential recycling due to sudden increases in nutrient levels (i.e., your baby-sitter decides to feed all those "starving" fish in your display reef tank, or your clownfish goes LOA without the discovery of a carcass...
.) Some other tests require more precision and accuracy, such as our Ca levels, alkalinity, and pH range.
pH is best determined via dedicated probe/meters, as there is a daily swing from a low in the morning of 8.0 or so in closed systems, to a high of 8.3 or 8.4 at the end of the photoperiod due to changing pCO2 levels in the tank (many threads in the archives on why this occurs). Knowing the range of this swing, and where your system is at a particular time of the day is more important than knowing the exact value at an unspecified time of the day. pH is but a single frame in a motion picture of the pH activity of any given day, and may have small ups and downs that are of little significance. The use of a test kit or graduated litmus strips is of little value unless you test about once every 2 or 3 hours to determine the daily range and swing.
pH is more important when using a CO2 driven Ca reactor, as it will help determine appropriate use of CO2 and whether your reactor is operating at peak efficiency. More details later if you're interested.
Ca++ and alkalinity values are important in determining if you're providing enough of these substances for optimal hermatypic coral growth (and clams and a few other calcifying organisms like calcareous algae, etc.), and will help determine at what rate you need to be replacing these substances to your system. Precision is important in this area, but again, you only need to be concerned with trends, not actual values. If you get a low value, retest, check your methodology, then make slow changes and retest every 3 to 4 days after making adjustments in your Ca and alk delivery rates. Once a week on this is usually sufficient to determine trends in this area.
Once a month testing for phosphates is sufficient, but more likely unnecessary. On testing for phosphates, it is difficult to get a good grasp on where you are with phosphates based on the actual level of phosphate in the water column. Available Phosphate for testing in seawater only represents inorganic phosphate, the same type that is easily taken out of the water column by biotic processes. Testing for phosphates is really kind of futile, a bacteria, algae, and most other living processes and a few abiotic processes remove most phosphate from the water column as soon as it appears. You really can only evaluate your phosphate levels on how the tank looks. If you allow detritus to accumulate in the system (i.e., detritus on the bottom, in the rock, under items in the system), then phosphate will become available as these substances decompose in the tank, and will trigger nuisance algal growths, in particular, cyanobacterial blooms and patch growths. It isn't so much that you have phosphates magically appearing in the tank, rather that detritus is decomposing within the system rather than being exported. Maybe we need a detritus test kit...
Low level nitrates are a once a week test if you have stony corals, as it is very important to keep these levels low to maintain adequate rates of calcification and skeletalization in
hermatypic corals. Slow changes in these levels may represent trends, especially in tanks without DSB or other means of exporting or completing the
nitrogen cycle. Important to have a good quality test kit for this value, and that you keep good records to watch for trends. Nitrates are always present in the water columns of closed systems, but may be at levels that our hobbyist's kits cannot detect, even with the low level (high sensitivity) test kits. If levels are below detection with low level kits, they are adequate for our purposes.
Salinity fluctuations are a big deal, and although there are not test kits in terms of reagent kit, testing your salinity should be an every day to every few days routine chore. This is especially true if you have an automatic topoff regimen, as all it takes to cause a potential disaster is for this mechanism to stick either on of off for a day. Daily testing of salinity/SG will eliminate the potential for extensive damage to your mini-biotope. This holds true for daily evaluation of temps as well, and I would recommend that you have a thermometer with a high and low alarm as cheap insurance for your tank.
Once every month to two months, evaluate your magnesium, as this will be a means of determining whether or not you're maintaining your proportionality of the conservative elements in your seawater. This test kit also needs to be quite accurate and precise, as small changes in Magnesium may represent large percentage shifts in both salinity and the conservation of proportionality in your water column.
Although there are a ton of other test kits out there, they are really of little use for most reef keepers, as most of the general husbandry tasks we perform will eliminate the need for these items (DOC kits, for example). You may want a copper kit if you have a used tank from an unknown source, (or you suspect sabotage from your competition...), but I'd just wait until you need one to buy one and get it fresh.
The kits that have definite titration endpoints will provide much more accurate and precise results than those that rely on interoperation of a color result. Many of the hobbyists level kits do not need precise repeatable results and will have large margins of error. Determine your needs based on what your expected results are and purchase accordingly.
HTH
