7 GH is upper level of soft..
Ive just copied this across from one of
@Essjay 's posts, Essjay was explaining how water companies say the water is hard, but that compares differently to what an aquarist considers to be hard/soft..
GH | ppm | Water company | Fish keeping |
0 – 2.8 | 0 - 50 | soft | very soft |
2.8 – 5.6 | 50 - 100 | moderately soft | soft |
5.6 – 8.4 | 100 - 150 | slightly hard | soft |
8.4 – 11.2 | 150 - 200 | moderately hard | soft to middling |
11.2 – 16.8 | 200 - 300 | hard | middling to hard |
over 16.8 | over 300 | very hard | hard to very hard |
The KH of your water is a measure of how well it can keep the PH stable.
With a low KH (less than 4), when acid builds up in the aquarium, whether its tannic acid or nitric acid, your PH won't drop accordingly straight away, because the acids need to 'eat' away at the KH first. Once the KH goes to 0, then the PH starts to drop ( or swing) to acidic levels.
With RO water, you're starting off with 0GH, 0KH and base (7.5?) PH
If you were to start adding acid (nitric acid comes from nitrates, or tannic acid comes from things that leach tannins like bog wood and leaf litter), then the PH would drop straight away and you'd be quickly down between 3 - 6 PH
Some fish are very happy in these soft water/acidic conditions. However, the challenge is to match the water at water change time. As a jump in PH is harmful to fish. That is why someone may wish to raise the KH of the water, in order to stabilise the PH.
Anything os possible, its just a bit more of a faff at water change time, where temperature matching isnt the only consideration.