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I know the subject of TDS is well covered here, but I’m looking for a little more info. I’ll try to give as much info as possible. I have an RO/DI unit that I use with an inline meter for measuring source, RO, and DI levels. My source(well) water consistently reads 185-205 PPM. It’s pH is rock steady at 7.8. It’s been a couple of years since I checked GH and KH, but when I last did, the KH was about 4.5 degrees, GH about 9. The RO/DI unit puts out 0 ppm DI, and about 10 ppm RO. Because it seems that it can help with inducing a spawn, on occasion I will mix in some DI to lower TDS in certain tanks. I typically will do larger water changes of 60-75% on all of my tanks, regardless. Yesterday, I decided to check TDS in each tank and was surprised to find that most were up around 400 ppm. Because my well water is so steady, I decided to change roughy 90% on several tanks. Logic would dictate to me that after the changes, the TDS should be somewhere around 220-240. After I finished the last tank, I went back and started retesting, only to find that the TDS has only dropped to about 330. So I’m looking for insight on what would cause the TDS to rise so high, and why such a large water change didn’t lower it more? I do use oak leaves in and alder comes in some tanks. Can the organic matter be part of the problem?

As far as I know, a TDS meater measures conductivity of the water, and organic matter - large molecules with little or no charge, do not contribute much to the conductivity.

What I think happend was, you went from TDS 400 to 40 after the waterchange, but then stuff went dissolving again.

I think you have rocks, or sand, containing chalk. CaCO3 more precisely. Now, in low pH (you will have pH lower than 7, as fish from soft water prefers that) CO3-- ions are instable, they will become HCO3-. Your chalk will slowly become calciumbicarbonate, which easily dissolves.

Perhaps you had a lot of this in the tank. However, I would assume a next 90 % waterchange would solve this, as the amount of Ca(HCO3)2 in the water is finite.

The problem is, however, that if the HCO3- is gone, there is still a lot of CaCO3 in the water.

What you can do is, measure the TDS, change water and measure it again. And after an hour again.

If it stays low, I expect it to be higher tomorrow, going back to the 400.

If this is the case, you will have to find the chalk and remouve it.

A TDS meter does read conductivity. It then applies a formula to that result and converts it into TDS. My continuous monitor offers me three choices, conductivity and then two TDS readings which use different conversion formulas. One drawback to the TDS setting I use is that it basically reads in 10 ppm increments. Fortunately my TDS pens read in single digits.

Bas Pels wrote: ↑10 Jan 2020, 08:41 As far as I know, a TDS meater measures conductivity of the water, and organic matter - large molecules with little or no charge, do not contribute much to the conductivity.

What I think happend was, you went from TDS 400 to 40 after the waterchange, but then stuff went dissolving again.

I think you have rocks, or sand, containing chalk. CaCO3 more precisely. Now, in low pH (you will have pH lower than 7, as fish from soft water prefers that) CO3-- ions are instable, they will become HCO3-. Your chalk will slowly become calciumbicarbonate, which easily dissolves.

Perhaps you had a lot of this in the tank. However, I would assume a next 90 % waterchange would solve this, as the amount of Ca(HCO3)2 in the water is finite.

The problem is, however, that if the HCO3- is gone, there is still a lot of CaCO3 in the water.

What you can do is, measure the TDS, change water and measure it again. And after an hour again.

If it stays low, I expect it to be higher tomorrow, going back to the 400.

If this is the case, you will have to find the chalk and remouve it.

After the large water changes, I couldn't have been at 40 PPM because they were done with my tap water which is about 190-200 PPM, not 0 PPM RO/DI. Considering that the tank was at 400 ppm before, a 90% change should have made it about 220-230. It took me about an hour to finish changing water, then I immediately rechecked the tanks in the order they were done. Most read in the 330-340 range. It would be hard to believe that if the water was 220-230 after the large change, that it would rise by 100 PPM in just 1 hour. My substrate is pool filter sand, which should be relatively inert. Most tanks have just a couple of random rocks and driftwood. I'm curious to see what the number will be if I do a 100 % water change, where 75% is my tap and 25 % is RO/DI.

TwoTankAmin wrote: ↑10 Jan 2020, 14:32 A TDS meter does read conductivity. It then applies a formula to that result and converts it into TDS. My continuous monitor offers me three choices, conductivity and then two TDS readings which use different conversion formulas. One drawback to the TDS setting I use is that it basically reads in 10 ppm increments. Fortunately my TDS pens read in single digits.

I'm using the inline 3 probe HM TDS meter that came with my RO unit. I just unplug it from the 3 water lines it's installed in on the unit(source, RO, DI) and hold one of the probes into the tank. I've tried all 3 probes to confirm accuracy across them. It reads in 1 PPM increments.