Aquarium Test Kits; What They Are Used For and Their Importance for Freshwater and Saltwater

It is always best to have as many different test kits as you can afford (as they apply to either freshwater or saltwater
I prefer to purchase my test kits separately as most master test kits duplicate test I do not need (high range and low range ph) and leave out ones I need such as kH and GH, or possibly Phosphate, Calcium, or Redox
Here is a list of FRESHWATER Test Kits, what they are for and in order of importance:
[1] Ammonia Test Kit; Ammonia is very toxic even at low levels, should be kept at or near 0. Ammonia test kits cannot discern between highly toxic ammonia (NH4) and only slightly toxic ammonium (NH3). Products such as Prime , Ammolock, and Amquel will convert NH4 to NH3, so your toxic ammonia level will not test accurately after using one of these products. Ammonium (NH3) will convert to NH4 at higher pH levels, which why high ammonia levels as per test are much more toxic at higher pH.
[2] kH & GH Test Kit; you will notice I placed this above a pH test Kit, this due to the fact that a poor kH will make keeping a stable pH nearly impossible. Also Calcium (as tested in GH) is more important to fish health than many aquarists give credit.
*You want to maintain a KH & GH of around 60 ppm (around 2-3 dKH & dGH) for Discus, Rams, Cardinals, and other Amazon fish.
*A KH & GH of 80 -100 for most tropical fish including Gouramis, Bettas, Angelfish, ECT.
*A KH & GH of 100-200 for Goldfish, mollies, Platties, Guppies, ECT.
*A KH & GH of over 200 for Rift Lakes African Cichlids and Brackish fish such as Monos and Scats.
For more information about Calcium and Magnesium, see my article: "Calcium, KH, and Magnesium in aquariums; How to maintain a Proper KH, why calcium and electrolytes are important".
[3] pH High Range OR Low Range Test Kit; you should only need one or the other based on the type of aquarium you keep.
Does not stress over being exact about pH, as too of many aquarists go WAY overboard here! For example Discus generally do best around 6.5, however I know breeders of Discus using water with a pH well over 7.0
I recommend a High Range Test Kit for Goldfish and livebearers which should have a targeted pH of about 7.5-7.8. Rift Lake African cichlids should have a pH of over 8.0
I recommend a Low Range Test Kit for general community freshwater fish (6.8- 7.4) or Amazon River fish (under 6.8)
[4] Nitrate Test Kit; this test is important to know when to change your water (along with KH), as well as how well your general long term tank health is going. Nitrates will also show you how well your plants are consuming nitrogenous nutrients, and if you are vacuuming the aquarium properly such as removing all the mulm. Some filters such as Under Gravel in particular, and Wet Dry and Canister filters can become nitrate factories if not taken care of properly.
For most freshwater fish a nitrate level under 40 ppm is correct or above 15 ppm for planted aquariums.
[4] Nitrite Test Kit; you will notice I put it last, not because it is not important, only that if budget is a concern or time for testing is a factor, this parameter mirrors ammonia often within hours or days. Nitrite should be at or near 0.
[5] Phosphate Test Kit; generally only necessary in planted aquariums where large amounts of "ferts" (fertilizer) are added or if you have uncontrollable algae growth. This test kit is often more important for ponds where rain water can wash debris with high phosphate content into the pond.
Your aquarium (or pond) should have phosphate levels as close to 0 as possible.
[6] Redox Meter; generally this water parameter will take of itself provided you perform proper aquarium husbandry including, regular water changes with properly ionized water, you maintain a GH and KH level of at least 50 ppm (I find 80 ppm better). This said, this is a good parameter to check if you are having problems with Cyanobacteria (Blue Green Algae) or if you are having unexplained problems and every other test is correct (providing you do not have an unknown disease). I recommend every aquarist be at least familiar with the Redox Potential, here is an in depth article about aquarium Redox: "The Redox Potential in Aquariums (& Ponds) and how it relates to proper aquatic health".
You can use Methylene Blue according to standard dosage to your aquarium water (remove the water for this test), and it stays a brilliant blue, you probably have a positive Redox. If the color dissipates at all (even the slightest) you probably have a reducing number. This test in no way replaces the accuracy of Redox meter, but it is a simple way to get a handle on your aquariums Redox health.
Your aquarium Redox should be around -300 mV.
SALTWATER;
The list remains similar and I will bypass descriptions that are in common with freshwater test kits.
[1] Ammonia Test Kit; this should be 0 as in FW
[2] KH Test Kit; you do not need a GH Test Kit for saltwater. KH (or alkalinity as is called in marine aquariums) should be over 240 ppm (13 dKH)
[3] PH High Range Test Kit; your pH should be between 8.2 - 8.4
[4] Hydrometer; this instrument tests the specific gravity of aquarium water (in simple terms the amount of salts). There are two types; the floating glass hydrometers which are more fragile, but also more accurate or the simple needle hydrometer that you fill with water. It is important with both types of hydrometers to clean these with white vinegar (then rinse thoroughly) to prevent hard water buildup which will then cause inaccurate readings.
[4] Nitrate Test Kit; Nitrates are best under 40 for marine fish aquariums and under 20 (even less if possible) for reef aquariums
[5] Calcium Test Kit; this takes the place of the GH Test Kit in freshwater.
Calcium is important to ALL marine life (this includes freshwater too as it is often sadly overlooked), however it is absolutely essential for proper coral growth in marine aquariums. Bio-availability is also essential, Sea Chem Reef Calcium Polygluconate is probably one of the best, most bio-available calcium supplements available for marine aquariums.
Your marine Calcium levels should be between 400-450 ppm
[6] Nitrite Test Kit; this should be 0 as in FW
[7] Magnesium Test Kit; Magnesium is an essential part of chlorophyll, which is necessary for photosynthesis, plants, including algae and the corals. Maintaining a correct magnesium concentration is very important, and is indirectly responsible for fast coral and calcareous algae growth by virtue of making the maintenance of correct calcium and alkalinity figures possible. Magnesium is depleted by algae and by the use of excessive Kalkwasser. Magnesium is what binds Calcium Carbonate in solution, if levels are low useable calcium levels will also drop. Poor choices of Marine Buffers can further exasperate this problem, this why I STRONGLY recommend Sea Chem Marine Buffer for adjusting alkalinity in marine aquariums (and EVEN freshwater aquariums) as this product has Magnesium, Calcium, and sodium bicarbonate in the proper ratios.
Magnesium levels in marine aquariums should be between 1200 and 1400 ppm.
[8] Redox Meter; as in freshwater, I recommend -300 mV, except Redox is more important in marine aquariums.
[9] Phosphate Test Kit; as in freshwater this should near 0. High phosphate levels in marine aquariums can play havoc with the health and growth of live coral (Phosphates can block the absorption of important minerals)