I recently purchased a TDS monitor to test my RO water etc. Yesterday I decided to check my tank (125 planted freshwater community) It read 189ppm with a flashing x10. This really surprised me so I did a 5 gal water change of straight ro water with no chemicals added (probably not enough of a change to make a noticeable difference out of 125gal) today the number read about the same (184).
I have a number of rocks in my tank that I got from fish stores and from my dad when he had this tank set up. the substrate consists of a 1 inch layer of aq gravel, clay substrate and laterite, and then a 1.5 in layer of straight aq gravel. So I assumed it was from the rocks. but then I tested my little 15 gallon tank which only has regular aquarium gravel and a clay like substance underneath (a planted aquarium substrate). Also I put laterite in the gravel at the recommended rate. And that TDS number reads much like the larger tank. Both use Ro water with the minerals replaced at the recommended rate of a medium hard level.
So what is making the water have such a high reading. What all is included in the TDS reading and how do I differentiate what is okay to be in that number and what could be harmful. In other words is all organic matter counted (nitrates, phosphates, which are necessary for the plants) then if nitrates are at 5ppm does that equate to 5ppm tds or is that not an equal exchange rate. In other words will a certain drop in the organic material cause a larger (or smaller, but not equal) drop in the tds readout.
I am less suspicious that it is the rocks since the reading was high in the little tank which does not have rocks, but how would I test that. I have had to use a couple of the plant weights on several of the plants that tend to develop smaller undergravel root systems because my synodontis would be so hard on them. Once the tank is better established, and come spring, I want to replace those with plants that establish better undergravel root systems. but could those weights be contributing to the problem. I have noticed that they will get a scaley, lime-like mineral build up on them.
nitrates 5ppm ammonia - 0 nitrite - 0 phosphates not tested but is not showing a sign of being a problem.
pH around 7.2 with slight fluctuations
KH 12 just tested that the other day and was surprised to see it that high. I had been adding sodium bicarb as a buffer at a suggested rate. when I did a little reading it looks like I could be adding half of what I was adding
GH was the same 11 or 12 a little higher than I thought it would be. I am working on lowering both of those numbers.
still it does seem like if I lower those two numbers KH and GH that the TDS will not drop by what I have read is the recommended amount of 500 ppm.