It's only in recent years that we've become aware of hardness as an issue. Though we can sometimes get to hung up on it. In very basic terms water is soft, hard or middling. Soft water fish shouldn't be kept in hard water and vice versa. There are some species which must have very soft or very hard water, but other soft or hard water species can cope towards middling as well. And where the water is very soft or very hard (as in this instance), even middling fish will not do well.
Yes, using RO water is an option. Even bringing the hardness down to say around 15 dH would open up more choices. That would need 3 parts tap water to 2 parts RO. But this is a commitment for the lifetime of the tank, and many fish keepers would not be willing to undertake this commitment.
The footprint of the tank is a significant factor here. Many otherwise suitable fish have to be ruled out because of the tank rather than the water.
I'll be honest and say I've not come across water this hard before. Having soft water myself, I'm at a loss as to what to suggest. Some of the suggestions made in other posts do have this hardness in their range, but right at the top end. We usually try to keep fish with our water nearer the middle of their range.
The rice fish mentioned do have this hardness in their range, but only just. There are other rice fish besides those mentioned which need softer water so make sure which the species in the shop tank is.
Least killifish (actually a livebearer, Heterandria formosa) has a range up to 20 dH. Tiger teddies (Neoheterandria elegans, another livebearer) might be OK. But these last two are quite hard to come by - I have never seen them in shops.
Pseudomugil furcatus or P. cyanodorsalis, two of the blue eye rainbows, would be OK - but there are other Pseudomugils which need softer water so the species in the shop tank need to be identified correctly.
Now I've exhausted ClownLurch's suggestions I'll stop there.
