Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Get each element separately, so that you add just what you want.
What units are you using when you say the GH is 16. 16 whats?
German degrees of hardness? That is hard water!
Parts per million? That is soft water.
No matter what you do, always show the units!
If you are using a pre-blended mix of what someone else thinks is the right fertilizers, how can you take out the stuff you do not need?
Hard water (high GH) usually has plenty of both Ca and Mg for the plants and fish (Yes, the fish use these minerals from the water, too), so there is no point in adding more.
Also, if you do have soft water, and want to raise the GH you need to know if there is an imbalance between the Ca and Mg, and add the right material, the one that is actually lacking in the tank. Not just add Mg to make the GH test read the right number. So, I am against any manufacturer creating one blend and saying it works for all, when it is so obviously wrong.
Some fish will keep on taking in the minerals and storing them in their body until they die from too much of something. Keeping soft water fish in high GH water can lead to this sort of problem. Cardinal Tetras are known for accumulating calcium like this.
To keep soft water fish when you have hard water:
Blend your hard water with distilled or reverse osmosis water until the GH is in the right range for the fish. If you think there is a problem with the Ca:Mg ratio, then correct it with the right material. Plants use Ca and Mg in a ratio of about 4:1. The water does not have to be exactly that, but sort of close is a reasonable target.
Then check the KH. This is carbonates and bicarbonates. The KH should be fairly close to the GH.
Once these 2 values are right the pH ought to be in the correct range for the fish. The pH is less important, and if it is not quite right, most fish will do just fine, as long as the mineral levels are right.
Some tap water is very skewed. Lots of GH and low KH or the opposite: GH is low, but the KH is sky high.
This is difficult to deal with, though the concept is the same:
Dilute the minerals until the one that is way too high is right. Then add the right materials to bring the low value into the right range.
Tap GH 1 degree, KH 10 degrees.
So, dilute this with RO @ 50/50 RO:Tap.
Net result GH is .5 degrees, and KH is 5 degrees. PH may have dropped.
So add some GH booster until the GH is at least 3 degrees.
If the pH is still way up, and you want it lower, then try filtering the water through peat moss.