Water change

[StevenF]

I think it's important to know things like this so that when people ask about heating water for water changes, we know that different countries do different things Like now I know that in the US, water for heating and water for other purposes are made separately and that it is safe to use water from a water heater for fish tanks.

Hoes in the US generally only have one water source from the water utility. That supplies cold water to all faucets and supplies water to the water heater. The water heater are steel tanks with zinc anode. The steel will not corrode. the zinc corrodes instead. Once the zinc anode is gone the steel will start to corrode. The tanks are sealed so dust, dirt, and other contaminant cannot get in the water.

Copper is generally not used in water heaters because it cost a lot more than steel. Copper is about $4 per pound while steel sells for about $0.40 per pound. Inmost homes hot water is not used for heating. Although water heated floors are a popular in some major remodel projects. Most homes have copper pipes although in new construction PEX plastic pipes are used. Prior to copper pipes lead and then zinc coated iron pipes were used. Most iron pipes and been replaced due to corrosion. Some old homes still have lead pipes that are generally replace during home repairs and remodeling projects.

Hot water is considered drinkable in the US. IT is basically driking water with a little bit of extra zinc added. Cold and hot water both have added copper due to the copper pipes. EPA limits drinking water to 1.3mg/L and zinc to 5mg/L Most homes have copper and zinc levels far below these levels. These levels are in place due to taste. these metal give the water a metallic taste. It is not harmful at these levels.

All animals and plants, including shrimp, need copper and zinc to live. Most fertilizers if dosed at the manufactures recomenation will only supply 0.001mg/l (PPM) of copper and zinc Most plants need significantly more. And most of it comes from your tap water. My water has 0.05mg/L (ppm) of copper in it and it does have a slight metallic test. Due to the test I have always used RO water. I had a lot of problems growing plants with Seachem Flourish coprhensive, and two other fertilizers. Mainly because all have 1ppm or less of copper and zinc. Once I identified the problem I solved it by making my own fertilizer. My first batch provided 20ppm of zinc, and 10ppm of copper. And since then Have generally not had problems getting good plant growth.

 

[Essjay]

The majority of homes in the UK use water filled radiators to heat the home. Most are gas powered though in areas where there are no gas mains, oil or LPG is used. A small minority of homes use ducted hot air (which can cause problems for allergy sufferers) or under floor heating.
So because there is already a boiler providing hot water for heating, they are also made to provide hot tap water in the same unit. Why have two boilers when you only need one?

It's the way the hot water is made that's the difference between types of boiler.
Combi, or combination, boilers provide hot tap water on demand from mains water. It is safe to drink hot water with this type.
Heat only boilers use hot water from the boiler flowing through a coil inside a cylinder of water to make hot tap water. This cylinder is topped up with water from a storage tank in the attic. With this type it is not safe to drink hot tap water - and it's because of the storage tank in the attic not copper.

If it's not safe to drink, it's not recommended for use in water changes.

 

[kwi]

I have not yet found anything that would fit this tap

Cooke & Lewis Gordale Chrome effect Kitchen Bridge mixer Tap | DIY at B&Q

This chrome finish bridge mixer tap from Cooke and Lewis' Gordale range brings a stylish finishing touch to your kitchen. It's suitable for high and low pressure systems.

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But there's still the problem that I can't use hot tap water and in January the cold tap water comes out at ~7 deg C (45 deg F)

Did you try unscrewing the aerator, you can then get a quick connect fitting that screws on in its place.
Something like this.

 

[Essjay]

I did manage to unscrew the end of the tap (I've just failed with all the others). But I still can't use hot water and have to boil a kettle to warm the new water (heat only boiler).

I also can't use a python type system to remove water from the tank as it would go down the sink. I should never have used the word bacteria in my husband's presence. He is now convinced that one drop of tank water will kill us all instantly with some vicious bacterial infection. So the idea of bacteria infested tank tank water going anywhere near the sink fills him with horror.
And before anyone suggests putting the hose into the garden, at the moment that's not an option. The garden is under water already, the flower beds are ponds and the lawn is like a wet sponge. The rain has even seeped in through the back of the garage so there's a pond in there as well. And there's more rain forecast.