04-25-2013, 11:06 AM
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Agree. With 40ppm nitrate in the tap water, you want to reduce this before it gets into the aquarium, and the link AD gave will go into that.
Mixing with rainwater will dilute nitrates proportionally as well.
Live plants, esp fast growing (and floating are excellent at this) will also help by taking up more ammonia so it doesn't get changed into nitrite and then nitrate by bacteria, since plants are faster than bacteria at grabbing ammonia.
I live in Greater Vancouver, in SW British Columbia, Canada. Our source water here is very soft, only 7 ppm which is less than 1 dGH. Same holds for the Pacific NW of the USA (Washington and Oregon, west of the mountains). Soft water fish thrive in our water, but plants need calcium and magnesium (the prime "hard" minerals) so these have to be added. I raise my GH up to 5 dGH or 6 dGH by using Equilibrium.