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     Frank I. Reiter <FIR at istar_ca> asks "So how hard *is* my water?"
     
     Sometimes the simple answer is right there in front of you! ;-)
     
     If you have RO water all you need to do is weigh the various compounds 
     you add to the water and you can easily compute the milligrams of X 
     per liter of water that you add. If you dose each chemical separately 
     such as calcium carbonate and magnesium sulphate (epsom salts), you 
     can precisely control the concentration of each ion species. Remember 
     not to add chemicals to the water you add to replace evaporation. Top 
     up, siphon off 10 gallons (or whatever), add 10 gallons and add 1 or 2 
     tsps of calcium carbonate. Remember that you also need to compute your 
     magnesium and potassium requirements as you do using PMDD. I tend to 
     dose that on an irregular basis so adding it during water changes 
     makes sense. I tend to develop potassium shortage symptoms.
     
     OK my turn to ask some questions: 
     
     Question (1) what is a good concentration for magnesium and potassium 
     in mg/l in a planted aquarium? Assume it has to be enough to supply 
     nutrient needs for 2-4 weeks in a strongly lit tank full of slow 
     growing plants without causing any problems by overdosing potassium. 
     (obviously some fast growth plants could soak up grams of nutrients 
     each week but who wants to chuck a pound of Ceratopteris each week?)
     
     Question (2) How many tsps of hydrated magnesium sulphate (epsom salt) 
     and potassium sulphate do I need for 10 gals of water? Ok I can figure 
     this out if I know the answer to (1) since I have a periodic table and 
     can weigh teaspoons. By the way don't forget to factor in the weight 
     of 6 H2O for each MgSO4.
     
     I'm kind of assuming that all the nitrogen requirements are being met 
     by fish food and wastes or from decomposition of substrate organic 
     components.
     
     Question (3) Are some plants sensitive to excess concentrations of 
     Calcium, Magnesium or Potassium? I've read in the t.f.h. Windenov 
     Tropica Catalogue, Aquariums Plants, A Complete Introduction, that 
     many species of Crypts do not tolerate an excess of Calcium ions. How 
     much is too much?
     
     Steve P in Vancouver