That's true, based off their natural habitat, however they adapt very well to hard water. The Ph would never pose a problem I have Extremely High PH, no problems with any of my fish.
Unfortunately, the OP has a pH of around 10. Most species that can adapt to hard water (like the gourami and marbled hatchetfish) won't do well in that high of a pH (as it's like crazy high for fish). Fish like that can only tolerate up to about 7.5 or 8. Usually pH isn't a large problem, but because it is so high in this case, it is a problem.
MyRainbowBettaFish, have you considered cutting your high pH, hard water with pure water? Something like rainwater or RO? It will really give you a lot more options.
Most of the farm raised fish will adapt to your source water and you can usually keep them long term without any problems, however, if you get wild caught soft water species or if you want to breed soft water species-Then you would need to change the pH/KH/GH of the source water by using R/O or rainwater.
Generally most fish you see at pet shops have adapted to the high pH and harder water than their native waters from years of being kept in the different chemistry-Most farm bred fish haven't been kept in their native chemistry for a really long time.
Do you know the KH/GH of your source water.
Your pH is 8-8.4 correct and this is after the 24h degassing without any additives-correct. You are using a liquid reagent type testing products and you don't have any type of water softening unit hooked up to the house that uses salt-correct....
With 10gal you are limited-what I would recommend-go where you plan to buy your fish and look at what you like. Find out if they are wild caught or farm raised and location reared/bred-Most of pet shops get farmed raised fish from Florida or the USA and these fish are usually already adapted, however, there are some exceptions.....
Then research-paying more attention to-adult size when fully grown, social needs, territory needs, food/feeding, temp ranges, compatibility and general tank size.
Since you don't want reproduction-I wouldn't worry too much about pH/KH/GH of your source water.
My well water-pH 8.8 with the KH/GH over 300-I call it liquid rock...lol...I keep soft water species without any problems, however, when I want to breed-I do have to soften the water by using rainwater-otherwise the eggs won''t hatch-but once the fry are roughly 4-6 weeks of age-I start using my hard water for water changes to get them adapted back to my well water. I have raised thousands of soft water fish like this over the years without any long term problems and they reach their expected longevity.