Relation of TDS to GH/KH

[darkdep]

For the last little while, one of my fry tanks has been losing fish at a very consistent rate, about 1-2 a day.  I've been so busy lately that I haven't had a chance to really think about it or do anything about it (wasn't such a big deal as I had well over 100 Spilonotus in there, and the freshly dead were recycled as food for the new Calvus). 

Anyway, this morning my daughter asked why the babies kept dying so we did a little daddy-daughter water test session.  I tested all my tanks with my TDS Meter which seemed to have very high readings; all tanks were reading between 600 and 950!

Thought maybe something had changed with the tap water.  Nope, a sample showed an expected 57.  So, new GH/KH kits were cracked open and got readings of 15 for GH and 6 for kH.  Here's where my question comes...I always thought that TDS was essentially a combination of gH, kH, and minerals such as Salt that couldn't be picked up directly. That means that for the fry tank in question:

TDS = 612
gH = 15 x 17.9 = 268.5
kH = 6 x 17.9 = 107.4

If the total of gH + kH = 375.9, how could I have such a huge TDS reading?

Yes, these tanks get weekly water changes of about 25%.

Any thoughts or knowledge to help explain?

[BigDaddy]

Your going to have other dissolved solids in the tank that don't register on kH or gH tests.  Sodium would be one example.  Your kH kit will give you the value of the bicarbonates in sodium bicarbonate, but not the sodium itself.  Since gH only measures calcium and magnesium... you won't register sodium with a gH test.

Given that sodium is a macronutrient, and it is also used as a water softener... well you get the idea.  I wouldn't be suprised if almost the balance of your TDS is mostly sodium.

[darkdep]

I would tend to agree with you BD.  I'm just a little surprised that so much sodium would be in there.  I mean, these tanks get nothing except a little salt with water changes (I don't add anything else to them).