Angelfish and Discus: Hard Water or Soft Water?

Should angelfish and discus be kept in hard water or soft water in the aquarium?

Q. I’ve noticed several ads offering angelfish and discus fish for sale that have been raised in hard, alkaline water. Because I have always read and been told that these fish require soft, acid water to breed, I’m not sure I understand the reason for keeping them in hard water. If I purchase such fish, is it best to continue keeping them in hard water or should I slowly change them over to soft water?

A. Like Amazonian cichlids in general, both discus fish and angelfish come from soft, acid waters in nature and they clearly prefer being kept under comparable conditions in captivity. However, they do share the ability of most cichlids to adapt to less than optimal water conditions. It is certainly possible to both keep and breed angelfish in moderately hard, alkaline water (pH to 7.6, up to 12 degrees hardness). One could probably maintain them as display fish without too much difficulty under more extreme conditions, but I doubt they would breed successfully. Assuming you decide to buy angelfish raised under hard water conditions, the decision to change them over to softer, more acidic water would thus depend both on how hard and alkaline your water is and whether you want the fish for display or breeding purposes.

I have seen discus fish both displayed and bred in moderately hard, alkaline water. However, both tasks are clearly more easily accomplished under softer, more acidic conditions. A measure of how much easier can be had by tallying up the number of professional discus fish breeders who have opted to invest in reverse osmosis water softening systems for their operations. Unless the chemical make-up of your tap water resembles the very hard, alkaline waters of Lake Tanganyika, you can probably display discus fish successfully without modifying the water chemistry.

Just remember that discus fish are sensitive to ambient bacterial concentrations and these are much higher under alkaline than under acidic water conditions. It may therefore be necessary to change water in a discus fish aquarium more frequently if they are kept in hard, alkaline water than would be the case if the water supply were soft and acidic. If you live in a hard water area and are serious about breeding discus fish, a reverse osmosis system would be a wise investment.

To answer your initial question, I suspect the chief beneficiaries of angelfish and discus fish reared in hard, alkaline water are retailers, who quite sensibly prefer to stock animals that will survive in their unmodified tap water, and that of their customers, who have the option of buying fish already acclimated to the conditions prevalent in their aquariums. Most of these customers have no desire whatsoever to breed either species, and much prefer to work with animals that can be expected to survive in the water that is readily available.

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Fish · Freshwater Fish