I'm in line with what has been suggested. But I would first look at the fish, as you have some incompatible fish in terms of water parameters and this should be examined before you start tampering with the water.
Livebearers including Platy
need mineral in the water, and pH above 7. What you have may be ideal for the Platy
now; you haven't given the number for the GH (in ppm or dGH) and that would be worth knowing. You can probably get this from your water supply people, they may even have a website.
The GH is just as important as the pH. Not only due to the relationship between these, but GH affects fish as does pH. I've already mentioned that the Platy
need this, but the rummynose are exactly the opposite; they need soft acidic water or they simply will not last long-term. Most rummys are wild caught and they occur in very soft, acidic water.
My suggestion is to decide on the fish, then adjust (if necessary) the GH and pH together. If you want more background on these and how they relate, have a read of this: Water Hardness and pH in the Freshwater Aquarium
We also have fish profiles, second tab from the left in the blue bar across the top of the page. Check the profiles for the mentioned fish, the preferred water parameter range is included for each species. If the name is used the same in posts as in the profile it will shade, as it did with Platy
, and here is the rummy, Hemigrammus bleheri
. The other species are in the profiles too.