Available water is strongly polluted with iron


I breed Betta splendens and recently am encountering a problem with my water. Here are the values for a start: pH 6; 16 °dGH; 3 °dKH; NH4/3: 0.5mg/l; NO2: < 0.1mg/l; NO3: n.n.; PO4: 0.3 mg/l; Fe: >> 1 mg/l; Cu: n.n.

The water contains very much iron (water is brown); now I wanted to ask how I can handle this.

there is actually only the possibility of completely deionizing the water - preferably with an ion exchange system rather than a reverse osmosis (R/O) unit, since iron tends to form gelatinous oxide hydroxides even in very weakly alkaline environments (that can indeed occur, e.g. by CO2 being driven out of the water). These gelatinous oxide hydroxides would very quickly and irreversibly clog the membranes. However, iron remains dissolved or reversibly bound to a cation exchange resin in an acidic environment, not depending on its oxidation state*.

Precipitating the iron followed by filtration probably causes far too much effort, also you would have to expect poorly filtrable 'jelly' again, depending on the precipitant you use.

* The following explanation refers to free iron: You can sometimes read that bivalent iron is quickly oxidized to trivalent iron in an aquarium and then precipitates. This is not correct. It mainly depends on the pH value whether iron precipitates or remains in solution. Both bivalent and trivalent iron will precipitate in alkaline environments, iron will however remain dissolved in both oxidation states in acidic environments (of course except poorly soluble salts such as the iron phosphates). For instance, ferric chloride (contains trivalent iron) is a common agent for etching electronic circuit boards, it is excellently soluble in water. On the other hand, solutions of, e.g., ferrous sulfate (contains bivalent iron) in water are also stable.

In acidic environments, dissolved bivalent iron is relatively stable against oxidation, but bivalent iron precipitated in alkaline environments will oxidize very quickly.

This explanation is just for the sake of completeness, as free iron is problematic in aquariums anyway. Fertilizers such as sera florena therefore contain chelated iron. This solves the question of precipitation.

And how can I raise the pH value to approx. 6.7?

You can use sera KH/pH-plus for this purpose.

Best regards

sera GmbH

Dr. Bodo Schnell


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