IMO, responsible aquarists monitor their water parameters by testing periodically, and proper water testing requires good test kits. I've used digital meters and reagent test kits over the years. Both are effective, and each has its place in the aquarist's toolkit. I currently use APIs test kits. Your local fish store (lfs) should carry them. If not, they're readily available online.
In order to understand the parameters of your tank water, you should understand the parameters of your tank water source. To that end, you should test your source water, whether it's municipal or well water, as well as your tank water. If you get all your water from a local fish store, check that too. If you filter your own water (municipal or well), make sure you set a baseline for the filtered water and its source before it's filtered. Changes made to any of these sources can affect the water parameters. The pH of water can change from one point of origin to the next and can also change over time (e.g., the pH of recently generated RO water will change over time in a storage container).
To be comprehensive with your testing, you should check levels of Ammonia, Nitrite, Nitrate, pH, Carbonate (KH) hardness and General (GH) hardness, and Phosphate. The API master test kit (shown at top) consists of a couple of test tubes as well as liquid reagents for testing Ammonia, Nitrite, Nitrate, and pH. Phosphate is a separate kit, so is the kit for testing the two hardness parameters. Combined, these kits will total four or five test tubes. You can purchase extras at your lfs or online.
Below is a test tube rack I purchased to help keep my tubes together and to facilitate their drying. The photo on the left is looking at the rack straight on. The right photo is looking down from above. The rack has two rows, one with spindles to invert the test tubes on and the other row contains hole slots to set the tubes down in. You will notice three syringes (left in left photo - syringe tubes in front row and accompanying syringe plungers in the back). You can pick syringes up at medical supply houses. I use these because they each hold just a bit more than 5ml, which is the volume of water each of the API tests requires. I bought the rack online for about $8.
Understanding your water and how its parameters can change will help you diagnose problems when they occur. For optimal fish keeping conditions, nothing replaces regular, frequent changes of water in closed systems (e.g., aquariums).
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