I am by no means an expert, but in my opinion, I would suspect that most, if not all SW test kits can
be applied to freshwater, but some may have limited use.
Sailfert are one brand I know of that develop a lot of SW tests, and some are suggested for FW as well....
Salifert Profi Test Kits
Salifert Profi Test Kits for reef, marine and advanced freshwater aquarists are some of the most precise and accurate test kits on the market. Kits are available for all aspects of the aquarium's chemistry, covering organic and inorganic pollutants as well as valuable marine trace elements to allow for accurate dosing when additives are required. The kits feature bold colour changes and good colour comparison charts and offer quick and easy water testing. Most are suitable for both freshwater and saltwater environments (unless otherwise stated).
This is taken from a large online UK retailer; Aquatics Online
. It states that some are Marine only, but I suspect that this is more due to the levels of each compound found in FW as opposed to SW and the range of the test kit, not because of an innability to work.
Here is my thoughts on those kits...
Iodine (Test range: 0.01 - 0.15ppm) & Strontium (Steps of 5 mg/L)
Not found in FW aquaria and not needed (ignoring debate about shrimp/iodine use
), or at very very low levels (below 0.01ppm).
pH (Test range: 7.4 - 8.7)
Range higher than normal FW water
Magnesium (Steps of 30mg/l) & Boron (Steps of 0.5 mg/L)
Resolution is not small enough for the relative small amounts found in FW.
These are my theories as to why they are considered only for Marine use as surely those developed for SW have to take into account many more
dissolved substances (salts) and higher TDS values that would be more likely to cause false results, than those that are found in FW. Put simply, FW is far "purer" than SW in terms of dissolved compounds.