[KrangDog]

So I've had this discussion with a couple of people privately, but haven't been able to figure it out yet so I was going to throw it out in the public domain.

I'm currently using RO water for water changes. It comes out of the RO system at 4ppm then I remineralise Ca and Mg 3:1 to 5dGH using CaSO4 and MgSO4, and front load macros bringing the water in my RO reservoir up to 18:0:38 using KNO3 and K2SO4 (PO4 is added daily via an autodoser to try to keep just a little in the water column when lights are on - it bottoms out quickly otherwise. It's not added to the RO reservoir).

If you add those numbers, you end up with 23ppm (Ca) + 8ppm (Mg) + 18ppm (NO3) + 38ppm (K) = 87ppm which sounds reasonable. However, by the time it's all added to the reservoir, the TDS comes out at around 205ppm. The reading that I've done suggests that SO4 is included in the definition of TDS. If you include SO4 in the calculation above (32ppm from K2SO4 and 85ppm from Ca and Mg), you end up conveniently very close to 205ppm. Theory and real world testing seem to agree very closely here. I'm testing with a calibrated Apera TDS20.

My macro dosing seems generally typical, with a 55% water change done weekly, it ends up at 10:0:21, and remineralising to 5dGH seems fairly common too.

So here comes the question. I've read of several people using similar macro dosing and remineralising Ca and Mg to similar levels and keeping their TDS around the 130-150ppm mark and report plant health issues when it goes above that range. How do they do it? The maths doesn't seem to add up. My tank runs at about 220-230ppm with weekly water changes and dosing as above. I'm fairly confident my TDS meter reads correctly, surely others aren't far enough off to account for the difference. Do I have a fundamental misunderstanding somewhere? Not too keen on getting into a discussion on the merits of targeting a specific TDS.

 

[Savetheplants]

220 is really good. The lowest I've been able to get is 340. I have very old driftwood and some Flourite. I think your right about the SO4. But I can see your frustration. How do they do it? Maybe we should start an experiment to figure this out. If you have a water column with no hardscape or substrate, I bet you could maintain a TDS of 130 to 150. So then what? You have to add stuff. I think Fluval Stratum would add less TDS than Seachem Flourite. Crumbling 13 year old driftwood like I have probably doesn't help either. It could be our maintenance isn't clean room standard. Maybe there's some mulm lurking in our substrate. Maybe our filters are a little too dirty. I share the frustration. How do they do it? Start with a water column and add one thing at a time to see what it does to the TDS. That's the only way to figure this out. Also some of these guys are running 0 KH. Maybe that's the difference.

 

[KrangDog]

I guess my point is, that if you're adding water to the tank that is 205ppm, there's no way to get down to 150ppm unless there's significant usage of nutrients by plants, or uptake by the substrate, or gassing off of some form.

Maintenance plays a part in keeping TDS down of course, but doesn't really explain why some people seem to be able to run tanks with very similar macro and GH dosing to mine (small variations between people/tanks of course) at 150ppm and below. To me, the maths just doesn't add up so I'm left wondering if my understanding is fundamentally flawed, or if different TDS meters can give such wildly different results. Or maybe SO4 breaks down into sulfur dioxide over time and gasses off...or something. I'm no chemist unfortunately.

 

[Savetheplants]

I guess my point is, that if you're adding water to the tank that is 205ppm, there's no way to get down to 150ppm unless there's significant usage of nutrients by plants, or uptake by the substrate, or gassing off of some form.

Maintenance plays a part in keeping TDS down of course, but doesn't really explain why some people seem to be able to run tanks with very similar macro and GH dosing to mine (small variations between people/tanks of course) at 150ppm and below. To me, the maths just doesn't add up so I'm left wondering if my understanding is fundamentally flawed, or if different TDS meters can give such wildly different results. Or maybe SO4 breaks down into sulfur dioxide over time and gasses off...or something. I'm no chemist unfortunately.

You make a good point about the TDS of the water going in. I remineralized RO water tonight and got 290 TDS. Then I checked the RO water and it was 70. Looks like my new water place needs to change their filters or something. So if I compensate for that, 290 minus 70 takes it down to 220. Then I could get to 205 by going down 1 dGH since my GH is 6. I could lower TDS even further by going to 4 dGH. The rest is probably in the KH. It has to be.

It really doesn't add up. I fertilize 40 ppm macros and there's always 15 ppm macro fertilizer left at the end of the week. You would have to add that to your pristine 150 TDS.

 

[Immortal1]

So I've had this discussion with a couple of people privately, but haven't been able to figure it out yet so I was going to throw it out in the public domain.

I'm currently using RO water for water changes. It comes out of the RO system at 4ppm then I remineralise Ca and Mg 3:1 to 5dGH using CaSO4 and MgSO4, and front load macros bringing the water in my RO reservoir up to 18:0:38 using KNO3 and K2SO4 (PO4 is added daily via an autodoser to try to keep just a little in the water column when lights are on - it bottoms out quickly otherwise. It's not added to the RO reservoir).

If you add those numbers, you end up with 23ppm (Ca) + 8ppm (Mg) + 18ppm (NO3) + 38ppm (K) = 87ppm which sounds reasonable. However, by the time it's all added to the reservoir, the TDS comes out at around 205ppm. The reading that I've done suggests that SO4 is included in the definition of TDS. If you include SO4 in the calculation above (32ppm from K2SO4 and 85ppm from Ca and Mg), you end up conveniently very close to 205ppm. Theory and real world testing seem to agree very closely here. I'm testing with a calibrated Apera TDS20.

My macro dosing seems generally typical, with a 55% water change done weekly, it ends up at 10:0:21, and remineralising to 5dGH seems fairly common too.

So here comes the question. I've read of several people using similar macro dosing and remineralising Ca and Mg to similar levels and keeping their TDS around the 130-150ppm mark and report plant health issues when it goes above that range. How do they do it? The maths doesn't seem to add up. My tank runs at about 220-230ppm with weekly water changes and dosing as above. I'm fairly confident my TDS meter reads correctly, surely others aren't far enough off to account for the difference. Do I have a fundamental misunderstanding somewhere? Not too keen on getting into a discussion on the merits of targeting a specific TDS.

Well, I am one of those that runs 130-150 ppm tds in two of my planted tanks and I likely have the answer for you. You have more _ added to your RO water than I do. "23ppm (Ca) + 8ppm (Mg) + 18ppm (NO3) + 38ppm (K) = 87ppm which sounds reasonable - but, from what I understand, ppm of __ does not directly translate to ppm TDS. dGH times a multiplier does seem to translate directly to ppm TDS.

In my case, I dose 20ppm CaSO4, 3ppm MgSO4, and 1dKH worth of K2CO3 to my RO water.
The 20ppm CaSO4 creates 2.798 dGH
The 3ppm MgSO4 creates 0.69 dGH
3.488 dGH * 17.86 gives me 62.3ppm TDS
My RO system shows 11ppm TDS in the drum before I add anything.
The K2CO3 adds another 18.5ppm TDS (tested separately as I could not find any solid into on the internet)
So, in theory my RO water with the above should test out at 91.8ppm TDS - I get 91-93ppm TDS on a consistent basis.
I add my macros to my aquarium after a water change. And over the course of 8-9 months my TDS testing data shows a before water change of 125-135ppm TDS. After a 75% water change I get around 100ppm TDS. I add my macros and I am back up to 115-120ppm TDS.

Now back to the earlier statement - I add less.
CaSO4 = 20ppm
MgSO4 = 3ppm + 2.94ppm from MgNO3 (5.94ppm Mg)
K2CO3 for 1dKH also gives me 13.93ppm K
6ppm KH2PO4 gives me another 2.47ppm K for a total K of 16.4ppm
15ppm NO3 from the MgNO3 completes my macros
Hopefully my info above is helpful.

 

[Deanna]

I wouldn’t try using a TDS meter to tell you what your actual ppm of total dissolved solvents is. There are several factors that skew the readings. The only true way to determine total ppm is the more complex weighing of the solids remaining after controlled evaporation.

TDS meters are designed and calibrated to certain representative solutions, with the typical choices available being either 500 (NaCl) or 700 (442) conversion factors. Getting a meter that also reads EC gives you a more consistent approach. There are also issues around range/accuracy specifications for a given meter.

The ions in the water have different conductivity strength, making averaging difficult to achieve with any precision. However, TDS meters are good for relative comparisons, particularly when using your own meter and tank. In North America, 500 scale meters are the most common so, as long as everyone’s meters are based upon 500 scale (and calibrated), and range/accuracy is similar, it will be possible to compare with other TDS meters, in relative terms, with some confidence.

Where I find value, for TDS meters, is with my own tank. I subscribe to the idea that a change of more than 10% or 50ppm, whichever is less, with a 500 scale TDS meter, is bad for fauna and can start to induce osmotic shock (we used to think that this was pH shock). Similar ‘shocks’ can also be harmful to plants, not least of the reasons being the introduction of instability in the nutrients. My 500 scale readings are typically below 100, but successful tanks at 400 and above are not uncommon. I think it more important to focus upon individual ion make-up, e.g.; Mg level and it’s relation to other ions, such as Ca.

You can start to learn more about TDS measurements at these links:
TDS 1, TDS 2, TDS 3, TDS 4

You can also do your own calculations, heavily based upon estimates, yourself or with this calculator: Lenntech

 

[KrangDog]

Thanks Immortal, that's helpful. But I'm not sure it fully explains it for me.

but, from what I understand, ppm of __ does not directly translate to ppm TDS

I suspect it does. According to the TDS Wiki, to determine TDS "Gravimetric methods are the most accurate and involve evaporating the liquid solvent and measuring the mass of residues left." So essentially they just evaporate all the water and measure the mass of anything that's left. So lets say you add 0.1g (100mg) of KNO3 to 1L of water, you'll have 61.3ppm(mg/L) NO3 and 38.6ppm(mg/L) K in that solution, giving a total of 100ppm(mg/L) of dissolved KNO3. Evaporate the water and you'll end up with 100mg(0.1g) of KNO3, and you've just measured TDS gravimetrically. I'd hope a TDS meter would read 100ppm. But again, I'm no chemist so could be misinterpreting something here.

3.488 dGH * 17.86 gives me 62.3ppm TDS

I'm not sure this 17.86 multiplier is correct. Pretty sure that's a conversion for dGH into ppm CaCO3. You're adding CaSO4. In normal circumstances, probably not an overly important distinction to make, but probably is important in the context of this discussion.

So if I was to add your amounts myself, adding 20ppm Ca from CaSO4 and 3ppm Mg from MgSO4 will add 23ppm from the Ca and Mg AND 60ppm SO4, so around 83ppm from that alone. Then you add your K2CO3 giving another 18.5ppm. And your water started at 11ppm. So I'd argue that you should be ending up with water 112ppm, so why are you measuring 91-93? Macros sounds like they're dosed into the tank as opposed to your RO reservoir so I've ignored those for now as it's a bit complex to do those calcs. The numbers I gave for macros are what I add to my RO reservoir, and given a 55% water change, is about double what I would quote if I was talking normal weekly tank dosing. Either way, you save a bit on SO4 by using K2CO3 and avoiding K2SO4

I wouldn’t try using a TDS meter to tell you what your actual ppm of total dissolved solvents is. There are several factors that skew the readings. The only true way to determine total ppm is the more complex weighing of the solids remaining after controlled evaporation.

TDS meters are designed and calibrated to certain representative solutions, with the typical choices available being either 500 (NaCl) or 700 (442) conversion factors. Getting a meter that also reads EC gives you a more consistent approach. There are also issues around range/accuracy specifications for a given meter.

The ions in the water have different conductivity strength, making averaging difficult to achieve with any precision. However, TDS meters are good for relative comparisons, particularly when using your own meter and tank. In North America, 500 scale meters are the most common so, as long as everyone’s meters are based upon 500 scale (and calibrated), and range/accuracy is similar, it will be possible to compare with other TDS meters, in relative terms, with some confidence.

Where I find value, for TDS meters, is with my own tank. I subscribe to the idea that a change of more than 10% or 50ppm, whichever is less, with a 500 scale TDS meter, is bad for fauna and can start to induce osmotic shock (we used to think that this was pH shock). Similar ‘shocks’ can also be harmful to plants, not least of the reasons being the introduction of instability in the nutrients. My 500 scale readings are typically below 100, but successful tanks at 400 and above are not uncommon. I think it more important to focus upon individual ion make-up, e.g.; Mg level and it’s relation to other ions, such as Ca.

You can start to learn more about TDS measurements at these links:
TDS 1, TDS 2, TDS 3, TDS 4

You can also do your own calculations, heavily based upon estimates, yourself or with this calculator: Lenntech

Thanks Deanna. I think you've solved this one for me. I found a setting on the TDS pen for a "TDS Factor" which I suspect is the conversion factor you talked about. It's currently set to 0.71 and I presume this is basically the 700 (442) that you referred to. Essentially a single EC unit is taken as equal to 700ppm (or in my case, 710ppm). It's adjustable all the way from 0.4 to 1.0. I imagine this is to deal with the issue of the different conductivity of individual ions.

If others are calibrated so that a single EC unit is equal to 500ppm (and if this is common in North America) it could be my reading of 205ppm would show as 145ppm if measured on a 500 scale. Based upon my closely aligned calculations and measurements, I'm tempted to consider my pen fairly accurate in the context of my tank, but perhaps not anyone else's.

I'll try changing the conversion to 500 tomorrow if I get some time and see if it reads around the 150ppm mark.

Thanks again, that's been driving me insane for a month now.

 

[Deanna]

it could be my reading of 205ppm would show as 145ppm if measured on a 500 scale.

No "could be" about it: 205ppm on a 700 scale is 146ppm on a 500 scale (500/700*205).

The confusion in all of this is the misleading assumption that the use of the acronym "ppm" with TDS meters is actual parts per million. On a 500 scale meter, that 146ppm is saying that the 146ppm is actual ppm if the only salt in the solution is NaCl.

 

[KrangDog]

The confusion in all of this is the misleading assumption that the use of the acronym "ppm" with TDS meters is actual parts per million. On a 500 scale meter, that 146ppm is saying that the 146ppm is actual ppm if the only salt in the solution is NaCl.

Yep, I understand that now. Maybe I'll start quoting EC from now on instead!