What is the magic of RO/DI Water?

Guyb93

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This needs explaining to avoid misinformation. And I am only responding with respect to your freshwater fish tanks questions, not marine.

Each species of freshwater fish has evolved to function in very specific water parameters; there are some species that have a wider tolerance than others, but for the sake of this discussion let's take the majority. Many, in fact just about all species that live in very soft water such as the Amazon basin, do not need any mineral in the water; they don't have it in nature, they don't need it in the aquarium. Ian Fuller [an authority on Corydoras species] keeps his wild caught Corydoras in pure RO. My tanks are all zero GH/KH because (luckily for me, as I like the soft water fish) that is what comes out of my tap--our water here is zero GH/KH. These fish thrive in this, because they are designed to by nature.

Other species that live in water that has calcium and magnesium (the basic GH minerals) need those, and if the tap water source is not sufficient, they must be added. Livebearers for example would die in my water without mineralization to increase the GH (and pH, this tends to follow when we are dealing with RO). There are other fish in between, needing some re--mineralization. But it all depends upon the fish species' natural requirements, and our being able to provide them. Not all fish can live well in the same water parameters.

The pH of pure RO water would be 7, neutral--neither acidic nor basic. Such water does not exist in the habitats of fish because water is a powerful solvent, and readily assimilates substances it comes into contact with, such as minerals dissolved from rock or organics causing acidic water. Rain begins as pure condensed water but as it falls it usually assimilates CO2 making it acidic; depending where it lands, it may assimilate other substances, changing its composition respecting GH, KH and pH.
Very interesting info I keep soft water fish because as tap water goes mine is soft maybe middling maybe more suited to Central American’s rather than south but still fit the Gh of 7dgh and always research the fish I buy to check but normally go on the rule of if they are South American it’s cool , never thought a Gh of zero could be inhabited for more than a few days , do plants still grow without the nutrients in the water ?
 

Byron

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Very interesting info I keep soft water fish because as tap water goes mine is soft maybe middling maybe more suited to Central American’s rather than south but still fit the Gh of 7dgh and always research the fish I buy to check but normally go on the rule of if they are South American it’s cool , never thought a Gh of zero could be inhabited for more than a few days , do plants still grow without the nutrients in the water ?

You are fine with this thinking. I tend to view soft and very soft water habitat fish as suited to soft water tanks. Given the GH of habitat waters for most of them in South America at any rate, the GH cannot be "too low" so you're fine.

Plants in the habitats tend to be rooted in the substrate of the water course if there are such plants in that watercourse (many have no plants) and the organic composition of the soils provides their nutrients, not the water. Marginal vegetation which is the more common are rooted in the forest "soil" which is extremely high in organics.

In the aquarium, we have a quite different situation; here we must provide the nutrients either in the water or the substrate, depending upon the plant species and how they feed. In such tanks, plants need to be more low-maintenance; the aquatic gardens of high-tech systems are out of place here, and the aquarist has to recognize that the fish must come first, and the plants second. I provide moderate (some would say low) light over my tanks, that was my decision from the start in order to provide a better environment for the forest fish I keep, and plants have to be able to manage with this. Minimal nutrient fertilizing follows, which is why I use Flourish Tabs for my swords, and sometimes a comprehensive liquid for the floating plants, depending upon the tank conditions. It is surprising sometimes how well such plants can do without more fertilizers, and the fish certainly benefit.
 

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