So my tapwater pH is 4.8

[Mozz]

The way I read it is the only ion you have in the water that is above the lowest measureable limit (I presume that's LOR) is zinc. I'm assuming that's coming from galvanised tanks?
It's very pure so will have no buffering capacity and the pH will drop with things like microbial activity on organic matter in the tank (a bit of leaf or blown in on dust)
As last line says = corrosion (hence the zinc coming off).
Someone more knowledgeable will suggest an appropriate way to buffer the water to protect the tanks and copper pipes and keep it prime for brewing.

 

[Mozz]

Just Googling a way to buffer and it seems like as little as 1-2 g of sodium bicarbonate per 1000 L from the baking section at the supermarket with raise the pH a couple of units which can be added every few months and after heavy rain .
Water will still have almost no salts for brewing which means you can add what you want to make any profile you need. Sweet..

 

[gone brewing]

The analysis shows total dissolved solids as 29ppm. The ions that are tested don't show anything at anywhere near that level so I presume that there are ions which weren't tested for that would add up somewhere around 29ppm. So I guess if you want a complete answer then you need to test for other ions.

However, if it was me I'd probably just test the mash pH and see if it's where I wanted it to be. At the end of the day that's what you want to get right.

And personally I wouldn't be too concerned about drinking tank water with a pH of 4.8. How old is the tank? Any chance of there being small amounts of organic matter (leaves etc) that sit on the bottom of the tank and decompose, leading to slightly acidic water? It sounds a bit gross but it's not that big a deal.

 

[mr_wibble]

I think those "other" ions are colourbond ions from the roof that's probably dissolving as it collects the rain.

The water tank is one of those huge galvanised steel jobbies, but it has a rubber liner, so the steel bit is just there to hold the rubber in shape.

There probably is the odd bit of leaf material that makes it through the input filter.

cheers,
-kt