Do You Know Your TDS? I do not know if everyone tests their TDS. I do. This does not mean it is important to everyone’s system, but if it is, I am sharing my recent experiences. My primary intent is to make a recommendation to those who buy RO/DI water or saltwater from a store: if want to start with pure water, test the Total Dissolved Solids (TDS) yourself. I assume if you have your own RO/DI system in your home or business that you check it, or have meters in line, but perhaps you do not? Background First off, I am blessed with several local fish stores (LFS) near me. I live 30 minutes north of Manhattan and there are many options in New York, Connecticut and New Jersey. Most I have seen are good stores with clean tanks, beautiful corals and healthy looking fish and inverts. I have generally found staff to be quite helpful and knowledgeable. Additionally, in my area, we start with excellent tap water. I have lived in three cities in Westchester County and the TDS of the tap water has been between 35 and 45. NYC and the Bronx I have seen reported about the same or even lower. I have read some local water reports and could almost use my tap water directly. The big problem with that is the Phosphate level runs very high at 2-3ppm (several years of API testing). My Observations Over the past couple of weeks I have visited four of my favorite LFS nearest me to purchase RO/DI water. In each case I was surprised by the TDS level of the water. In two cases, I called before going in and was told the level was 0. One seemed surprised I asked, saying “it’s RO/DI water.” Here are the numbers from the four stores in ascending order: TDS 7 14 41 253 Okay, 253 is nuts, but it is correct. I will talk more about that in a minute. Two things jumped out to me right away: if you buy RO/DI water or saltwater from a store, you should test the TDS yourself, and these LFS are likely using the same water in their own displays, and in a couple of these cases this was a little scary. The two lowest samples I collected, the 7 and the 14 TDS, I further tested for Phosphate. In both cases the result was barely detectable. That led me to use them as intended as change water for some dry rock I am curing. The LFS that tested 14 told me they do replace their filters every month since they go through a ton of water and I believed them. I understand that filters, membranes, resins, etc. can degrade quickly with constant use. The sample I got that was 41 TDS I dumped out. It was higher than my tap. I speculated at first that maybe they filled my jugs with tap. I gave them the benefit of the doubt after speaking with them. The person felt terrible and offered me store credit. Their store is pretty new and I considered maybe they had not yet changed out their filters since the initial setup. They followed up with me and noted they had replacements on order. I still wonder how long they have used it in their own system. The 253 TDS number was shocking and baffling. I tested multiple times while also testing other known sources to confirm. I obviously dumped this one right out. After much speculation, my best guesses are the person (a newer employee) somehow gave me the waste water from the system, or the collection containers themselves were heavily soiled. Perhaps it was some fluke or mistake. It does not really matter. The point is if I had just used it without testing I may have had issues. Conclusions I firmly believe that it is ultimately my responsibility alone whenever I add anything to an aquarium. That is why I test no matter what I am told. I urge everyone to test all pure water they purchase or make with a TDS meter. A good TDS meter can be purchased for $20-30 and the test is fast and easy. Just dip the probe in the water. Some areas have terrible tap water to begin with and there is only so much an RO/DI system can accomplish. In these areas it seems crucial to test. My research so far has shown me two things: the TDS of the RO/DI water available from local stores was regularly higher than I expected, and none of the stores themselves (to be fair the staff who helped me) knew the actual TDS of their RO/DI water and, when I asked, none knew the TDS of the tap water they started with. I said in the beginning of this thread, I consider all of these stores good, clean, honest and helpful. RO/DI water sales are a very small part of these operations and their system’s filters degrade over time. They probably all go through a lot of water. My point is not to bash anyone (and I do not care to name names). I only want to suggest anyone regularly buying RO/DI water or saltwater from a store should test the water themselves. If you buy saltwater, then ask them if you can test their RO/DI water or buy a small amount if you feel weird asking. Your Turn Water quality is pretty important to the life in our systems, so I felt I should share my own limited observations. What do you think? I am honestly unsure how concerned people really are with the purity of their water. These threads seem to suggest many are, but what is the reality. Do a lot of folks use tap? If you buy your water do you test the TDS level? Does the store you buy from test it? How high a TDS number is acceptable to you? If you have your own RO/DI system, do you test? How high do you let it go before swapping out filters? I would love to hear other’s thoughts and experiences. Thanks!