Hard water is more suitable for ponds as it acts as a buffering against variation swings of the PH, this is when acid or alkali changes occur, without hard water the PH variation could be more excessive and the pond life could be at risk.
Fish can become stressed when there is a dramatic change in a pond environment, a bit like when you are introducing new fish to your pond, you have to make sure that the temperature of the pond water is on par with the water you have transported your new fish in, this can be done by floating the bag in the pond until it has acclimatized you can also mix some of the pond water with the water in the bag a little at a time.
Letís point out that if you are just keeping goldfish then testing for hardness is not really necessary, if you are going a bit upmarket and keep koi its well worth investing in a quality test kit.
Harder water in your pond also assists in controlling variation in metallic impurities which can become a problem to your fish. It also helps a fish to regulate its own metabolic processes and reduce stress. Hard water can assist string algae or blanket weed to form, in very hard water ponds excessive algae builds up on the sides of ponds.
Remember, keep an even balance, you then get healthy and happy fish.
Return from water-hardness to water quality