Why is the water in my aquarium so acidic? - Quora

Another similarity in blackwater aquariums is the water is soft and acidic.
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In the wild, water with low alkalinity always has low hardness. So adding acids to an aquarium might lower the pH out of the alkaline range, but this water won't resemble anything a fish would encounter in the wild. Instead, you can lower the pH and hardness by using water purified by reverse osmosis. This process removes almost all of the dissolved minerals in that water. You can add reverse osmosis water to aquarium water to dilute it, which lowers the alkaline and hardness.
API pH Alkaline Acidic Test Kit Freshwater Aquarium Water Fish Plants 250 Tests
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The normal trend for pH in a saltwater system is downward, or more acidic, which stems from the addition of acids to the aquarium. These acids come from several sources, the primary ones being: (1) excess (CO2) from respiration caused by lack of sufficient gas exchange, (2) nitric acid from biological filtration (nitrification), and (3) organic acids from metabolic wastes. pH measures how acidic or alkaline (basic) your aquarium water is, on a scale of 0 to 14.
Photo provided by FlickrpH DOWN lowers pH to make freshwater aquariums more acidic for fish and plants
Photo provided by FlickrYou can of course use a pH buffer on its own to createacidic conditions in a soft water aquarium and skip the peat filtrationentirely.
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1. Soft water exhibits a stronger tendency towards pHinstability than hard water. All aquaria become more acidic overtime, but in soft water aquaria this trend can be very rapid. Since fewfish will tolerate rapid changes in pH, frequent pH tests and the useof chemical buffers are an important aspect of maintaining a soft wateraquarium.One of the paradoxes in freshwater fishkeeping is thatwhile most fish naturally from soft water environments will thrive inhard water aquaria, the reverse is almost universally not true. Tetras,Barbs, Gouramis, catfish and Angelfish are allexamples of originally soft water fish that are routinely andsuccessfully kept in hard water community tanks. But Livebearers,Central American cichlids and Rift Valley cichlids almost never adaptto soft and acidic water conditions. In other words, if all you want isa mixed community tank, then hard and alkaline water will allow you tomix Platies, Neons and without problems.4. Filter bacteria work best in hard, alkaline waterconditions. In very soft and acidic water, filter bacteria may notwork at all, forcing the aquarist to use less efficient methods offiltration, such as the use of zeolite.3. Photosynthesis. The effect of plants on pH iscomplex because aquatic plants use two different sources of carbon,dissolved carbon dioxide and bicarbonate ions. Dissolved carbon dioxideforms carbonic acid, which lowers pH. As plants photosynthesise, theyremove this carbonic acid, allowing pH to rise. However, in mostaquaria the actual concentration of carbon dioxide in the water is verylow, which is why aquarists concerned with plant growth need tofertilise their tanks with addition carbon dioxide. If there isinsufficient carbon dioxide for their needs, plants will switch tousing up bicarbonate ions. Bicarbonate ions are a major part of thealkalinity reserve in the water, that is, the ability of the water toresist changes in pH. With the bicarbonate ions taken away, the waterloses some of its buffering capacity, and the pH drops. The overallresult is that rapid plant growth coupled with insufficient carbondioxide and/or bicarbonate concentration can lead to dangerously fastacidification.Before looking at how to set up a soft water aquarium, itis essential to first explain the problem of acidification. All aquariahave a tendency to become more acidic over time. Acidification iscaused by four main things:Soft water aquaria are different. By definition, softwater contains few dissolved minerals, and as a consequence thealkalinity reserve is very low. There's nothing much in soft waterto mop up acidic chemicals, so these will accumulate and lower the pHbetween water changes. In other words, soft water aquaria are subjectto faster and more severe drops in pH than hard water aquaria.