This is an excellent article by Rosimeri Tran, published in the Central CA Koi Club November 2016 newsletter, about research she did on garden hoses.  Click on the picture or title to read the content.

A few months ago I was made aware buy a club member and friend that they had performed a water change, like they do weekly, nothing different, no new Koi, no changes what so ever, except they had purchased a new garden hose to fill up their pond. It was only after this water change and new water that they lost almost all their Koi. Seeing that they did nothing different and the water tested ok with no ammonia, nitrite or nitrate, pH was okay, it led them to believe something inside the garden hose was the cause of the Koi deaths. So I decided to do some investigation because I just couldn’t understand how a garden hose could kill fish, be-cause remember as kids we drank out of garden hoses and assume some kids still do, how is it possible that something lethal could be lurking inside the garden hose.

But it is true; in my investigation I found that not all garden hoses are meant for Koi ponds or even for us to water our vegetable gardens and fruit trees. Some of these garden hoses contain lead inside the hoses which is to prevent the inside of the hose from breaking down; they are treated with chemicals, which are harmful to humans and Koi. There was a study made by The Ecology Center which tested 21 different garden hoses purchased from Home Depot, Lowes, Target and Wal-Mart, the result of the investigation was that one third of these hoses tested positive for high levels of one or more chemicals such as lead, cadmium, bromine (flame retardant), chlorine (which indicates the presence of polyvinyl chloride, or PVC, phthalates and bisphenol (BPA). Not to mention that more than half were made of PVC, which has been proven to be a significant source of exposure to chemicals known as phthalates, which are used as plasticizers. Of these testing positive for phthalates, these were known to be banned from children’s toys from the Consumer Product Safety Com-mission, but yet were found in these garden hoses. A good portion of the hoses, 4.5% contained brominated flame retardants, this chemical has been linked to cancer, reproductive problems and impaired fetal brain development.
Read what came out of this study:

What this means is that, assuming your garden hose is made of toxic plastic chemicals, as it sits outside in the sun those chemicals concentrate in the water. When the Ecology Center researchers tested water from a hose that was left out in the sun for two days, they found:

BPA levels of 0.34 to 0.91 ppm, which is three to nine times higher than the safe drinking water levels used by the National Science Foundation (NSF),

DEHP (a phthalate) levels of 0.017 ppm to 0.011 ppm, which is two times higher than federal drinking water standards

Generally speaking, the study found that PVC and vinyl hoses were the worst in terms of leaching phthalates and BPA, which makes sense since phthalates are used to make plastics flexible. Those with copper fittings contained the most lead. In 2012, the researchers found even worse results when they tested 90 garden hoses, including BPA levels of 2.3 ppm and DEHP levels of 0.025 ppm. (For that study they also found that many gardening products, including gloves, kneeling pads, and tools, contain chemicals of concern as well.) The Ecology Center reported on the garden-hose findings.

Now to eliminate this problem it is advised to purchase “drinking water” safe hoses, which are available such as polyurethane, or natural rubber hoses. The label should indicate it is safe for drinking water and at least say it is lead free. Read the labels on the garden hoses, I have one that Seng just got it is a Flexon Heavy Duty 50’ 5/8” garden hose, nowhere on the front does it indicate anything, but when you turn it over on the back side of the paper it says, “WARNING: DO NOT DRINK FROM THIS HOSE:” so if you never read the back of the garden hose, which I assume most of us don’t, you would not know this. Another recommendation for the garden hose it to keep it covered and out of the sun, because water builds up in your hose, with the heat it increases the leaching of chemical additives into the water. So once you turn the water off, that warm water will contain the most chemicals, so letting the water run out of the hose before using it until the water is cold temperature is another way of reducing the chemicals.

These “drinking water” safe garden hoses are available; I found some online via Amazon.com, also found at Target and at camping and RV stores. So do your research before buying a new garden hose, because al-though you have not had any problems as of yet, you don’t know what chemicals are lurking inside that garden hose that may very well kill your koi.