Ah but, 75 to 85 ppm of what? In the UK, it could be ppm Ca (calcium), ppm CaO (calcium oxide) or ppm CaCO
3 (calcium carbonate). This is how annoying hardness is with all it's units!
For example, my water supplier gives my tapwater as 34ppm Ca, 85ppm CaCO
3, 5.95 degrees Clarke, 4.74 German deg and 8.5 French deg. And moderately soft.
If your 75 to 85 ppm is CaCO
3, it is roughly the same as mine which my water supplier classes as moderately soft.
The 60 to 80 ppm Ca band is moderately hard.
And if it's CaO, that's somewhere between moderately soft and moderately hard.
If you could find out what the ppm are it would help enormously. If it helps, home test kits measure ppm CaCO
3.
My tapwater pH is 7.6 freshly drawn. It drops to 7.4 on standing 24 hours. If those ppm are in CaCO
3, your water is very similar to mine. Because hardness is more important than pH, I can keep soft water fish easier than hard water fish.
And if the hardness is ppm calcium carbonate you are quite likely to have low KH as well. This doesn't affect fish directly but it can allow the pH to drop. KH is a measure of the amount of carbonate; carbonate is a buffer. It reacts with acid and keeps the pH stable. Nitrate, the end product of the nitrogen cycle, is acidic as are lots of other things secreted by fish. The natural tendency of a fish tank is to become slowly more acidic. The presence of carbonate in the water stops this. But with low KH, there isn't much carbonate to start with and it gets used up. Once it's gone there is nothing to prevent a pH crash. If you do have low KH you need to do water changes every week without fail to replenish the carbonate.
I found this forum back in its previous incarnation in 2006. I was a bit lazy with water changes and discovered my pH had fallen off the bottom of the tester scale - it was below 6.0. This is where I found out about the implications of low KH. I have had no problems now that I do water changes at least once a week without fail. Learn from my mistakes