Calcium, Magnesium, TDS

I, Mr. AnalRetentive, have recently purchased yet more Lamotte test
kits, this time a Hardness test kit (Total, Ca, Mg) and an electronic
Total Dissolved Solids (TDS) tester.  The results of testing our
current tanks are thus (all values mg/l):

                      Total    Ca     Mg      TDS

100g Discus tank:       48     40      8      120
100g 'Bow tank:        104     88     16      140
120g 'Bow tank:        100     80     20      160

(BTW, the total hardness values agree almost perfectly with a Tetra GH
test kit using 10 ml of water (0.5 dGH per drop, 1 dGH = 17.8 ppm)).

I noted that _The_Optimum_Aquarium_ shows Ca:Mg ratios of about 2:1 in
natural streams.  Our measured ratio is shows much more Ca than this, 
leading me to want to boost the Mg with some magnesium sulfate.

I've heard that TDS is a useful measurement (I believe Oleg Kiselov
made that statement many years ago).  It has one or two (or maybe
more) meanings to aquarists (correct me if I'm wrong here): the simple
meaning is a quick way to determine hardness.  Higher TDS readings
indicate more "total" hardness.  Since most hardness is Ca/Mg
hardness, TDS hardness is a quick way to electronically measure "GH". 

TDS in mg/l is really a conversion of the conductivity (reciprocal
resistance) of the water and measures the total amount of ions in the
water.  Calcium and magnesium ions are (should be?) predominate with
other contributions from sulfate, nitrate, phosphate,
potassium, etc.

How would the chemist gurus interpret the reading above?  The discus
tank has almost the same TDS as the 100g Rainbowfish tank, yet the
Ca/Mg hardness is very different.  The water is treated much the same
(Dupla stuff) with the main difference being more GH added to the 'Bow
tank.  We didn't check nitrates and the discus tank has a higher
bioload (the measurements were made right before a water change).  I
should do them again.  

Could TDS in conjunction with Ca, Mg, and Fe tests give us a better
picture of the nutrients available to plants?

George in Sunny Northern Colorado