Is My Water Too Hard For Discus?
by Carol Roberts

Probably not!  Juvenile discus actually grow faster in moderately hard water.  Most places in North America have water suitable for growing out juvenile discus. 

In the wild, discus live in very soft (low GH) and acidic (low pH) water.  Domestic discus can be kept in soft to moderately hard water (GH 3 to GH 14). Most discus are raised in a pH above 7 and many hobbyists report success with a pH of 8.5 or higher. 

Discus do need water with stable parameters. They do not like large drops in pH.  If you have moderately hard water or well water you may need to “age” your water to stabilize the pH.  You can age your water by aerating or agitating it for several hours to release dissolved carbon dioxide. Aged means the pH has reached equilibrium.

How can I find out the hardness and pH of my water?

You can call your municipal water department and ask for a printout or you can buy a couple of inexpensive test kits.


Electronic meters will measure pH and conductivity more accurately than the test kits.

What is conductivity and hardness?

Conductivity measures the ease with which an electrical current will flow through the water. Usually, the more dissolved solids that are present, the higher the conductivity. Conductivity meters do not measure total dissolved solids, but are used to estimate dissolved solids in the water. The dissolved solids present in aquarium water might include salts of calcium, magnesium, sodium, potassium, iron, manganese, silica, barium etc, etc.

Conductivity and hardness are important when breeding discus. The “shell” of a discus egg is a semi-permeable membrane, similar in many respects to the membrane in an R.O. unit. Water from the aquarium can pass into the egg at a controlled rate to provide the unborn discus fry with oxygen. The rate is controlled automatically by the “osmotic pressure” of the water. The higher the dissolved solids, the higher the osmotic pressure. High osmotic pressure reduces the rate at which water flows into the egg, so the fry are effectively suffocated.

Hardness (GH) is a measure of all calcium and magnesium salts present. Carbonate hardness (KH) is a measure of just the carbonate and bicarbonate salts. The dissolved salts that cause hardness are by far the most common ones in water. So, generally (but not always) an increase in conductivity is caused by an increase in hardness.

Do I need to use RO water?

A Reverse Osmosis filter removes minerals and impurities from the water. Most domestic discus of all ages do quite well in dechlorinated tap water.  If your discus are laying eggs and the water is GH 8 or higher you will need an RO unit to remove minerals from the water.  You will get a higher hatch rate in GH 3 to GH 6. 

Never use pure RO water.  It is too soft and the pH can drop dramatically. You can mix a percentage of RO and tap water to make stable water with a GH 3 – 6. After the eggs have hatched you can stop using RO water and convert the tank back to tap water by using your regular aged water for the daily water changes.

Should I lower my pH?

No! What is important is a stable pH.  Better to consistently keep your discus in a pH of 8.0 than have the pH swing from 8.0 down to 6.5 and back up to 7.5.  Hard water with a high pH is notoriously difficult to adjust.  Strong acids must be used to chemically reduce the pH.  Many times the reduction is temporary and the pH bounces back in a few hours.  You can strip most of the minerals from the water with an RO unit and then add acid to lower the pH.  However, a miscalculation could result in a pH crash and dead discus.

What is the best water for discus?

You do want to learn about your water.  Test the pH from the tap and test the pH of a cup of water left on the counter overnight.  Test the GH and KH of your water.  Log the results so you have them to refer to.

You can successfully raise tank-bred discus in nearly all of the domestic water found throughout North America.  Generally, if water is fit for human consumption it’s fit for your discus tank.  Chances are the best water for your discus is your own dechlorinated tap water. You may have to stabilize the pH by agitating it or you may be able to use it straight from the tap.  What’s best for your discus?  Fresh, warm, pH stable water is best for discus. Water changes promote growth, increase appetite and maintain health. Don’t worry about altering your water chemistry. Do a water change instead.