"pH" is a term used by chemists to describe the acidity or basicity of a
solution. A measure of 7.0 is neutral: neither acid nor base.
pH ranges measured below 7.0 are considered acidic. pH ranges above
7.0 are considered basic.
Sudden changes in pH are very stressful to fish.
Four terms describe the
total amount of dissolved mineral content that is present in the water:
Buffering Capacity = Total Alkalinity = pH stabilizing ability = Hardness
of your water.
Some tropical fish such as Discus are very sensitive to water hardness
and need very soft water. Marine tanks and some African Cichlid tanks
usually need hard water.
Monitoring the pH alone allows you
to detect when a change has taken place, but it provides no information
regarding the stability of the pH in the tank environment.
The dissolved mineral content (alkalinity) affects the ability of your
water to hold its pH values steady. Those minerals which directly affect
the pH stability (buffering capacity) of your water are:
carbonates and bicarbonates of sodium, potassium, calcium and magnesium
as well as hydroxides of sodium and potassium.
The natural progression of most aquariums is toward the acidic pH range.
Organic decomposition turns your freshwater tank more acidic over time.
In marine tanks in particular, the removal of minerals by
the organisms as well as the topping off of the tank with
mineral-stripped water destabilizes the pH and causes it to fall. If a tank
is depleted of mineral content for any reason, the ability to keep
its pH stable is compromised.
Raising the pH and keeping it up requires dissolved mineral content
(total alkalinity/buffering capacity).
Water with low levels of dissolved minerals (soft water) such as
de-ionized or Reverse Osmosis water (and some naturally occuring waters)
will be very hard to stabilize. The pH will continually drop. This is
a huge challenge for Marine tanks and some African Cichlid tanks
and requires the continual monitoring
of total alkalinity and the addition of the requisite minerals
to keep the pH stabilized at a high enough level.
Freshwater fish usually prefer water with fewer minerals and lower pH.
However, even established freshwater tanks need some buffering capacity
(dissolved mineral content) to remain in the safe (though acidic)
pH range. A pH below about 5.5 can kill the beneficial bacteria in the
tank and destroy the biological filtering process. This leads to
high levels of ammonia which eventually kill the fish.
The toxicity of any ammonia in your tank is also directly affected by the
pH of the water. Water which has an acidic pH (below 7.0) will have
little TOXIC ammonia (although ammonia is actually present).
Water whose pH is alkaline (over 7.0)
will immediately turn benign ammonia into TOXIC ammonia with potentially
Regular testing of your tanks pH and hardness will allow you to
detect and correct problems before your fish are adversely affected.
Freshwater environments are usually buffered to a pH of between 6.5 and
8.0. South American cichlids and most tropicals like a more acidic
tank -- 6.5-7.0. Some african cichlids and mollies like a pH that
is more alkaline -- in the 8.0-8.3 range.
Reef tanks require a pH of 8.0 to 8.3, and an alkalinity of 2.9 to
4.0 meq/L (8 to 11 dKH) is recommended, with 3.4 meq/L (9.5 dKH)
Fish-only marine tanks should maintain pH of 8.0 to 8.3 and
an alkalinity 3.5 to 4.5 meq/L (or 10 to 12 dKH).
In marine tanks, do not allow alkalinity to go below 8 and
don't exceed 12. This will provide proper pH stability, and still not
cause heavy precipitation of important minerals, such as calcium and