Unfortunately, the only realistic way to soften water is to mix tap water with RO water. Ignore any product that says adding it will soften water. The problem with them is they remove calcium (which is what makes water hard) and replace it with sodium or something similar. What you need to do is remove minerals of any kind, not just swap one for another.
Having said all that, slightly hard is not a problem; very hard is the problem! I have water classed as slightly hard and keep soft water fish with no problems. My pH is around the same as yours too.
So, fish that grow 7 to 10cm that would be OK in your water in a 90 litre tank, though it's only 60cm long (ie a tall tank not long shallow tank). That last statistic does limit the fish you can have as a lot of 7 - 10 cm fish need a longer tank.
If you could find some, a pair of
thick lipped gouramis or possibly
banded gouramisA pair of Apistogrammas. The species to look for would be
A. cacatuiodes,
A. agassizii or
A. borelli as these can tolerate slightly harder, more alkaline water than most other apistos. Or
Nannacara anomalaPeacock goby/gudgeon though it might be too small for what you want.
Fish in the genus Aphyosemion, Fundulopanchax etc though as they are jumpers you need a tightly fitting lid. These are killifish, there are several genuses. Some of them are so-called annuals, that is they live only one year.
Just a few suggestions