Question for TDS experts. I do not quite undestand

[bela]

Hi there.

I am relatively new in the use of a TDS meter and there is something that I do not quite get.

My tap water is around 75 ppm which I presume is on the soft side.

I have nothing calcareous in the tank right now, just some wood, lava rock and inert "normal" gravel.

Can any of these things make the water go from 75 to almost 200?

Has any of these things absorbed some compound (I used to have dead live rock in this tank) and now they are slowly releasing it back to the tank (I am thinking about phosphates or silicates or the like)?

How can it be that if I make an 80% water change, the TDS measurement remains almost the same?.

I mean, if I change 50% of the tank´s water, shoud not the TDS go down 50% also? 

Unless there is something in the tank that keeps "dissolving and dissolving" and maintaining the TDS up.

I am not worried about the TDS per se, but I think the high TDS is the responsible of the algae problem that I am suffering.

Any feedback is appreciated.

Thanks

 

Edited September 11, 2023 by bela

[Guppysnail]

This explains them in detail as well as things that will change your water TDS

 

[TOtrees]

What are you using for water changes? If you're putting in "pure" water like what you'd get out of a RO/DI system, then yes a 50% change should reduce TDS by 50%, since the new water would be close to 0 ppm. But if you're using tap or well water, it's certain to have lots of dissolved solids. This varies a lot by region. 

Also, there is lots that a TDS pen will measure that we have no other way of quantifying, so even if you can't account for it that doesn't mean the pen is wrong. 

Finally, ANY amount of salinity will play havok with tds meters in freshwater systems. I'm overstating/overinflating/oversimplifying the issue, but not by much. And I don't remotely understand it myself, to be honest. 

[nabokovfan87]

So... there's a few things going on here.

On 9/11/2023 at 12:04 PM, bela said:

I have nothing calcareous in the tank right now, just some wood, lava rock and inert "normal" gravel.

Can any of these things make the water go from 75 to almost 200?

A.  What is the TDS in your tap? (75)
B.  What is the TDS in the tank itself (200?)
C.  What volume of water are you changing and how often?
D.  Are you topping off with tap water or RO water?
E.  What is the GH and KH of the tank vs. tap?

Ultimately TDS means "stuff" so you can use the fancy word document tools and just replace "stuff" where need be to better understand your water and what you're seeing.  If you have TDS consistently climbing, what that means is that you have old tank syndrome setting in.  This would be where PH drops, GH goes up, and you have a big difference in your tap vs. tank.

Planted tanks will use up minerals, evaporation will raise up TDS, adding anything to the tank will raise TDS, and of course you have stuff in the tank already which do dissolve over time into the water.  TDS is of value right before that water goes into the tank and after it comes out of your RO system.  Even if you're measuring the tap and you say it has 75 TDS, without GH, KH, and a water test report it's very difficult to know what is actually in that water.

My last question and bit of info here, why do you want to measure TDS?  What is the goal for tracking it?

 

On 9/11/2023 at 12:04 PM, bela said:

I mean, if I change 50% of the tank´s water, shoud not the TDS go down 50% also? 

If you do a 50% WC with RO water you will dilute it.  I would expect ~40% or so of the TDS to be removed.  Then you add in dechlorinator and other ferts and that will raise it back up.

On 9/11/2023 at 12:04 PM, bela said:

I am not worried about the TDS per se, but I think the high TDS is the responsible of the algae problem that I am suffering.

In terms of algae....

1. GH should be about 2x your KH if possible.
2. Test your phosphates for high phosphates.
3. If you're seeing organics and mulm and detritrus en masse, then you need to siphon a bit better and clean your filtration.