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Re: Hard Water

     >Rats- I was hoping to start a fascinating thread a la *what color is clear
     >water in a white pail* but George is a man of few words...  ;-)
     
     Ah, but what is the color of clear water in a RED pail? 
     
     >I use distilled water when replacing evaporated water to avoid mineral
     >build up in my tanks. Does anyone else do this or feel that this is
     >neccessary?
     
     A very clever discussion/explanation on this topic can be found at:
     
     http://www.cco.caltech.edu/~aquaria/Krib/Chemistry/h2o-changes.html
     
     and I will attempt to answer, based on Ron Wozniak's discussion above. 
     My apologies to Ron if I blow it.
     
     By topping off your tank with tap water, you are adding water + 
     dissolved minerals to replace the pure water that has evaporated; over 
     time, your tank have a higher mineral content than your tap water due 
     to evaporation, even if you do regular water changes.  Eventually, the 
     amount of minerals gained by topping off will equal the amount removed 
     by water changes, but what will this long-term concentration be?
     
     Okay, lets assume a 100 gallon tank filled with tap water containing 
     300 mg/l dissolved solids.  2 gallons are lost every week to 
     evaporation, which is replaced with tap water.  This amounts to (2 
     gallons x 3.78 liters/gallon x 300 mg/liter) = 2,268 mg of minerals 
     added every week.  So according to Ron's handy table, with a 20% 
     weekly water change, in the long run this would result in an 
     additional 9,072 mg, or  24 mg/liter, over and above what the aquarium 
     started out with.  This would result in a final concentration of 324 
     mg/liter in the aquarium.
     
     According to Ron's table, it would take 8 weeks to reach 80% of this 
     concentration, 11 weeks to reach 90%, and 14 weeks to reach 95%.
     
     With only a 10% weekly water change, the final concentration would be 
     354 mg/liter, while a 25% weekly water change would result in 318 
     mg/liter.  Break even (no increase in dissolved minerals) would 
     require 50% weekly water changes.   These figures assume no net uptake 
     of any of the minerals by the plants or fish.
     
     Of course, the higher the evaporation rate, the higher the increase in 
     minerals, and vice-versa.
     
     In conclusion, as long as you do regular water changes, it seems it is 
     probably not necessary to top off with distilled/deionized water, unless 
     you are trying to maintain some VERY STRICT water quality parameters.  
     Regular water changes can solve/prevent a LOT of aquarium problems.
     
     Golly, I hope I got the calculations right!
     
     Kind regards,
     
     Mark