Ooops, dropped to 5 ppm TDS

[TCMontium]

Something very unusual happened. The water in one of my aquariums has the TDS reading of 5 ppm, which means the device is measuring a conductivity of approx. 9 microsiemens (my TDS-Meter has a seemingly inconsistent multiplier which seems to multiply higher conductivity water with bigger values than lower conductivity water, ranging from maybe x1.6 to x2.4).

I only use RO water for water changes and my tanks usually have "TDS" ranging from 18 to 60 ppm (mostly depending on the frequency and percentage of water changes I make). I never had an aquarium with a "TDS" lower than 15 ppm because my RO water reading range from 9 to 18 ppm and I do not make 100% water changes. And I didn't do a water change in this particular aquarium in the last 3 weeks either! Could it be that the plant growth (Salvinia mainly) lowered the TDS/conductivity so much?

And I have a pair of A. mendezi in the aquarium. Is lower than 15 microsiemens unnatural and harmful for this species? The female has been laying eggs several times already and I have never seen fry. Maybe it is because of the low conductivity making embryo/larval developement problematic?

 

[Mike Wise]

First question: when was the last time you calibrated your conductivity meter? As for A. mendezi, conductivity below 10µS/cm is typical for them in the wild.

 

[TCMontium]

First question: when was the last time you calibrated your conductivity meter? As for A. mendezi, conductivity below 10µS/cm is typical for them in the wild.

That's good to know.
I never calibrated it, bought it 2-3 years ago, it was very cheap too (maybe 10-15 dollars). But is it measuring wrong if when I test it with bottled water, the TDS reading is nearly the same as the given value written on the bottle? I did this with several brands and the reading match with the bottle info with +-10 ppm accuracy. Of course if the calibration doesn't effect only the accuracy, but consistency and "the range of accurate measurement" too, then I surely need to calibrate it for low conductivity values. Or buy a more advanced conductivity reader to measure RO water.

 

[Mike Wise]

My meter is calibrated regularly (monthly). Even expensive meters 'drift'. Without calibration your meter could be off considerably at low conductivity.

 

[gerald]

When comparing nature vs aquarium, keep in mind that interstitial water (between the sand particles in the stream bed and seeping out of stream banks) may have higher mineral content than the surface water where scientists usually take water samples. Fish and inverts in streams where the measured conductivity and hardness is ultra-low may actually have access to mineral ions in seeps and stream-bed water that conventional sampling doesn't detect.