TDS stands for Total Dissolved Solids or Total Dissolved Salts. TDS is the measurement of all inorganic and organic dissolved solids in your water. This includes your important things such as calcium, magnesium and carbonate that you can measure using your kH, gH, pH kits. However, it also includes all the other dissolved minerals, salts and more. Basically anything that is too small to be filtered out. This is why testing your TDS on top of all the other tests is vital to your keeping your water at ideal levels, because all these other elements and ions can have affect on your whole tank. It is these other minerals and salts that can make it possible to have a High TDS reading even when your gH ad kH readings are low.
Have you ever seen or been warned of dropping electrical products such as a toaster or hairdryer in water and getting electrocuted? It is not because water can carry a current, because it can’t. Pure H2O is not able to carry electrical charge. However our tap water is not pure water, in fact only distilled water is pure. All other water is a solution with many dissolved minerals, salts and more in them – these dissolved ions all carry either a positive and negative charge. This makes the water able to conduct electricity. Depending on where you live, where your water comes from, and what condition your pipes are in, tap water can have a TDS reading up to 500PPM. It can be even more if your pipes are in bad shape.
TDS Meters are generally easy to find – Most hardware stores will sell a variety of them. TDS Meters basically measure how easily a current will run through your water. The easier a current can run through, the higher the concentration of TDS (and you have in your tank. Water becomes electrically charged at 50PPM. The only downside of a TDS reading is that you don’t know exactly what ions are present. You just know that they are there (That is why we do separate testes for gH and kH). This is why generally it is easier and better for your tank to keep your TDS levels under control.
High TDS will affect water clarity, so if all your parameters are great and your filter is doing well and your water appears cloudy it is because your TDS is high. This decreases Photosynthesis. Also because of the high concentration of positive and negative ions whether they are dissolved or suspended (TSS) the temperature in your tank will rise because these elements absorb heat. Also high TDS might mean the presence of high concentrations of lead or other toxic compounds. TDS will also affect your ph slightly. High TDS water is usually slightly alkaline while low TDS will be slightly more acidic. This means Low pH species are usually also low TDS species.
The simplest way to control your TDS levels in your water is to always use 0 TDS RO water for water changes so your TDS does not drastically change. Always Check TDS levels of your tank and the water you supplement with. Rainwater and RO water and even bottled distilled water that is meant to be “pure” can have TDS in them. I read an article about how the FDA requires “purified” water to have a TDS of 10 or less yet when tested a lot of bottled water brands some will have even up to 180. So regardless of where you get your water, always check the TDS so you know what your putting in your tank. This will make it easier to keep the level as constant as possible.