Rex's Planted Tank Guide

Your Source Of Information For Planted Tank Aquariums, Lighting, CO2 & Fertilizers

Rex's Take on Water Chemistry

So long as you are able to drink your tap water without harm it can be used to grow plants. Some plants may not do as well in extremely soft or hard water but most plants are very adaptable. A pH anywhere in the 6.4-8.5 range is fine. A general hardness, gH, of 3-15 degrees and a carbonate hardness, kH, of 3-12 is fine.

If your water is very soft and you are using CO2 injection you want to be sure and add something to get your kH up to at least 3 degrees to prevent large pH swings. If your gH is very low then you may need to provide your plants with calcium and magnesium. If in doubt ask questions. Don't try and become a “water warrior” and constantly fight your water. If you have hard water plan accordingly. Unless you have a lot of time and money it's not worth fighting the water. Now I will tell you that I should be keeping Apistos and Discus as I have very soft water. But it's much easier and cheaper to make soft water hard than it is to make hard water soft.

One thing to avoid is the use of any chemical buffers to adjust the pH. Doing so usually results in what I call chasing the dragon. You add the buffer, the pH changes, then in a matter of minutes to days rebounds to the former level. So you all more buffer and repeat the process. This is very harmful to your fish. If you need to lower the pH then use CO2 injection. If you need to raise the kH and/or gH (and very few of us do) then use something like baking soda, for kH or calcium carbonate for kH and gH.

If you need to raise the kH of your water then you can use plain old baking soda, not baking powder but baking soda. One teaspoon of baking soda will increase the kH of 50 liters of water (13 gallons) by 4 degrees (68 ppm). Or you can use calcium carbonate which will also raise the gH. Two teaspoons of calcium carbonate will raise both the kH and gH of 50 liters of water (13 gallons) by 4 degrees (68 ppm). If your water has low gH you will also need to add magnesium to it.

The best way to do this is with Epsom salts. ½ of a teaspoon of Epsom salts will raise the magnesium level to 7.03 ppm in 10 gallons of water. Recommended levels of magnesium are 5-10 ppm. If your water has a gH of over 3 degrees you would not normally have to add magnesium. For a very in-depth look at water chemistry take a look at this site. The one problem with using calcium carbonate is that it is very hard to dissolve. So if you need to raise gH use Epsom salts and calcium chloride instead.

To sum it up. To raise kH use baking soda. One teaspoon will increase the kH of 50 liters of water (13 gallons) by 4 degrees (68 ppm). To raise gH use calcium chloride and Epsom salts. Add them separately or you will end up with calcium sulfate which takes a very long time to dissolve.

0.79 grams of calcium chloride and 0.33 grams of Epsom salts will raise 10 gallons of water about 1° of gH.

Planted Tank Water Chemistry – Q & A

1. How can I lower my pH?

  • The real question is why do you want to lower your pH? Almost all aquatic plants will do fine in most any water conditions. There are a very select few that require softer water. There are two ways to lower pH in a planted tank.
    The first is through the use of CO2. This lowers pH but does nothing for water hardness or the TDS (Total Dissolved Solids). The second is though the use of peat. Peat will lower pH and lower hardness. It does however stain the water. The last option is a RO (reverse osmosis) unit that will remove all hardness and TDS from the water. However these are expensive to run and waste a LOT of water.

2. I have read that (insert name of plant here) requires soft water. Is that true?

  • Maybe. There are a small handful of plants that do better in soft water. Since most of these plants are plants that are hard to keep and propagate they are in short supply. Again, the majority of aquatic plants have one basic requirement for water conditons...wet.

3. How can I soften my water?

  • The only two ways that work for softening water for a planted aquarium are to use a RO unit or peat.

4. My house has a water softener. Will this harm my plants/fish?

  • It can. You have to understand how a water softener works. Go to this link to find out. That article covers salt based water softeners. A potassium based one works the same way they potassium based water softeners are not very common. When the water is softened the calcium and magneisum are removed and sodium added. That makes the water “softer” but does nothing to the TDS (total dissolved solids). The problem being is that plants and fish that require soft water also require (normally) low TDS. The sodium has been reported to cause problems with some plants.


Work with the water you have. Not against it. Adding acids to lower the pH will almost backfire and raises the TDS.