Strange swing in tds, RO water

[riioKen]

Hi guys, I have a 330L planted tank since September. I did a 2 month dark start with tap water (around 400ppm).

After 2-3 months, I switched to 100% ro water remineralized with caso4 and mgso4 at wc volume (150L, around 15g, 23ppm Ca and 12g, 7ppm Mg).

Usually my ro water tds before remin is ~20ppm, after 115-130ppm.

The tank tds after wc was usually 180-200ppm, while at wc day was around 280-290ppm.

Now, it's 2 weeks that I get strange values.

The ro water before remin tds is the same as before, but after, same amounts of salts, instead of 110-130, I get 60ppm.

The tank after wc, instead of being ~200ppm, is 60-70ppm and at wc day sits around 100ppm.

The results are paler colors and some melting leaves. Fish seems unaffected.

Any suggestions?

 

[LMuhlen]

Could be something wrong with your TDS pen or your scale... Does the TDS pen have some sort of TDS-conductivity switch, or some way to calibrate?

Could be that you were using anhydrous salts (more concentrated) and they absorbed humidity from the air, becoming more diluted.

Not sure what else could be happening.

 

[riioKen]

Could be something wrong with your TDS pen or your scale... Does the TDS pen have some sort of TDS-conductivity switch, or some way to calibrate?

Could be that you were using anhydrous salts (more concentrated) and they absorbed humidity from the air, becoming more diluted.

Not sure what else could be happening.

I thought it too, but I've just calibrated it at 50ppm and when testing my tap, the result is correct, the measure of RO water without remin is the same too, I think that the pen is working fine.

Idk how to check the last part.

The salts are CaSO4 .2H2O and MgSO4•7H2O

 

[bazz]

The ro water before remin tds is the same as before, but after, same amounts of salts, instead of 110-130, I get 60ppm.

It could well be what @LMuhlen has described,

Could be that you were using anhydrous salts (more concentrated) and they absorbed humidity from the air, becoming more diluted.

are you keeping your salts in good quality airtight containers, I can imagine it being quite hot and humid in Italy at this time of year?
I'm no chemist but I know that some anhydrous salts are used as desiccants therefore I would imagine CaSO4 .2H2O and MgSO4•7H2O between them can absorb enough water to reduce the Ca and Mg concentration by weight to 50%.

 

[Hanuman]

Now, it's 2 weeks that I get strange values.

The ro water before remin tds is the same as before, but after, same amounts of salts, instead of 110-130, I get 60ppm.

The tank after wc, instead of being ~200ppm, is 60-70ppm and at wc day sits around 100ppm.

The results are paler colors and some melting leaves. Fish seems unaffected.

Are these new salts that you bought or are they from the same batch you were using before?
More possibly what is occuring is one of two things:
1. you are using new salts and they are not the same hydrates as you were using before;
2. most likely IMO, user or machine error. Your scale could be damaged or you mistankenly weighed the wrong amounts. Your TDS pen is fine since it's measuring the same TDS as before of your RO water and you are seing a reduced TDS as well after remineralisation. The problem lies in between, namely the salts and/or the weighing.

are you keeping your salts in good quality airtight containers, I can imagine it being quite hot and humid in Italy at this time of year?
I'm no chemist but I know that some anhydrous salts are used as desiccants therefore I would imagine CaSO4 .2H2O and MgSO4•7H2O between them can absorb enough water to reduce the Ca and Mg concentration by weight to 50%.

I very much doubt this is due to the salts having absorbed water. One can expect some hydration to occur with time but IMO unless you accidently dropped a glass full of water inside your salt bags I don't see this being the reason. I live in a country where humidity reaches 90++% during certain periods of the year. I have yet to see any of this dramatic TDS drop when remineralizing my tanks with the same salts that I have had for over 4 years now.