Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
What is the GH and KH of the tap water and tank water?
KH is a measure of carbonates. Carbonates are one of the most common buffers of pH in the aquarium. When carbonates are high, the pH is almost always high. When carbonates are low the pH is more free to vary, and other things in the tank can control the pH.
Organic matter from driftwood, fallen food, fish waste, fallen leaves and other things generally have the result of lowering the pH. If the carbonates are low, then these other things can make the pH low.
Yes, it is possible for organic acids, including tannic acid to lower the pH that much.
Tannic acid itself will usually be seen as a tinting in the water, but there are other organic acids that might not be so noticeable, and any of these, present in even small amounts can lower the pH.
Here is a quick test:
Run a couple of jars of tap water.
Test GH, KH, and pH.
Add baking soda (just a pinch) to one jar, stir well and test again.
Allow both jars to sit out overnight and test again. You might need to stir the baking soda jar again.
a) Pure tap water might have any level of carbon dioxide in it, and allowing it to air out overnight will bring that CO2 into equilibrium with the air. This is the 'control'; what the water will do if you do nothing to it.
b) Adding baking soda is adding carbonates. This should raise the pH. The results should be permanent, but in a quiet jar the baking soda might settle out somewhat if it was not fully dissolved. It would not settle out in an aquarium, there is always some water movement to keep it dissolved. On the other hand several organisms in the aquarium use carbonates, so the level can drop over time, and the carbonates need to be replaced.
You can run the test longer, several days to a week, if you want, or you can vary it by starting with aquarium water and adding baking soda to that.
Anyway, if this test shows you that your water needs carbonates to maintain the pH that you want you then need to figure out how much. The most common way to add carbonates to the aquarium is baking soda (sodium bicarbonate) but some people do not want to add sodium to the tank. You can use potassium bicarbonate (A food item).
Here is one recipe:
1 teaspoon of baking soda, added to a 29 gallon tank, will raise the KH by 2 German degrees of hardness. In my tanks this raises the pH from 'off the chart low' to 6.2.
That recipe can be altered any way you want, any tank size, any amount of change in KH. For example, altering just one factor in the recipe:
Double the tank size = half the change.
Double the dose = double the change.
Combining several changes:
If you want to raise the KH by 4 degrees (double the change) in a 15 gallon tank (half the volume) use 1 teaspoon.
I have used that recipe to figure KH changes for tanks ranging from 10 gallons up to 88 gallons, and KH changes from 2 degrees to 5 degrees.
Do these changes slowly so the fish can adapt. For example, make the KH rise by no more than 2 degrees at any one time. Let them adjust to that for a few days then you can raise it again.
Other ways to get a similar effect, test these in a jar or bucket:
Add something to the filter that will dissolve slowly, and add minerals to the water.
Coral sand, Oyster shell grit (sold for caged birds like Budgies), limestone gravel or sand and similar materials will do this. Cuttle bone or seashells in the tank or filter will also do this. I use a nylon stocking for the sand sized materials.
These are not really the best treatment by themselves. Every water change would create swings in the mineral levels which are not good for the livestock. I use these materials to keep conditions stable, but when I do water changes I make the new water correct before adding it to the tank. I will add baking soda for KH and GH booster (Calcium and magnesium) to the water for certain tanks, and circulate it until it is well dissolved and the GH and KH test shows the water is correct for that tank. I do not go by the pH. Through the week the organisms that use each of these minerals may use some, but the coral sand etc. in the filter slowly dissolves so the tank stays stable.