Water Butt.

I'm not sure about the green. But there could be other problems with using water from a butt.
Is the water free from contamination - do you live in a pollution free area?
Is the water collected from your roof - is the roof clean and made of something that won't contaminate the water, no bird droppings on the roof etc?
As this will be mineral free water, are you adding remineralisation salts?

Provided the water in the butt is free from contamination, it's the lack of minerals that would cause a problem. There are very few fish, and none in the trade that I know of, that can survive in water with no minerals. The water from your butt (I hope there are no American's reading this, it sounds very iffy  ) will have zero GH and zero KH. You could either add remineralisation salts, the kind you have to add to RO water, or mix it with your tap water if that is very hard.


But unless your tapwater is very hard and you want soft water fish, or your tapwater has high nitrate and you want to keep nitrate sensitive fish, I would just use tapwater treated with a dechlorinator. I would be too afraid of contamination if we collected water from our roof (assuming that's what your butt collects water from) as we have crows, rooks, seagulls, woodpigeons and blackbirds reguarly sitting on our roof and doing what birds do. If it anything like the state of the path next to the house, I wouldn't use any water that's been in contact with that.

If you trust the quality of the water butt's water, you can mix it with your tapwater to make it softer. As you have hard water, a mixture would give you enough minerals for soft water fish. Or use a tap/RO mixture.