I hope you don't take offense at this but to me Endlers are just expensive mini guppies. Like guppies, (or platys) they have a one-sided personality: Greedy. I get the whole "live to eat" thing but sheesh, take a break fercryinoutloud!
It just seems to be a fight to push softwater fish into a hardwater environment.
It does, but imho it's made into a bigger deal than it needs to be, unless we're talking about the extremes at either end of the limits. Hard water has been blamed for everything from fin rot to lethargy, lack of appetite, lack of coloring, organ failure, shortened life spans....blah blah blah ad nauseum. I'm sure I'll catch grief for this but I don't believe "hard" water is the issue, but rather what's in it that makes it hard? Technically my water is considered hard: 10-12dGH in the summer, sometimes as high as 14 in the winter, TDS hovers around 350 or above. But if I believe the city's water report there's nothing in it in high enough concentrations that should affect fish. Most of our water in the US is hard and while we fish nerds might obsess over it I'd guess the average person never gives it a second thought and in most cases can successfully keep healthy, active fishies.
With plants? Maybe it makes more of a difference, as I have a hell of a time with those. But like fish, most are "supposed" to be kept in softer water. So what's a hard water guy to do? (I know, RO. Pfffft!) If I limited myself to only hard water fish I wouldn't need to ask for suggestions here, I'd just throw a dart at the list of 20 or so (yes, exaggeration. but not by much) and go from there.
FWIW, the 8-year-old neons in my 20L are absolutely stunning, and I'm not particularly a huge fan. I rescued them from one of the big box stores not expecting them to make it but man have they thrived! The CPDs look great too but I'd be happier with them if they'd knock off the breeding. Freaking eggs everywhere and I simply can't ignore them.
Not that I'm advocating trying to give our aquatic friends anything less than the absolute best quality of life. Like I mentioned I'm willing to soften the water if necessary, but being a low maintenance kinda guy I'd prefer to avoid it.
It's too bad there's not a freshwater puffer other than peas. They'd be perfect for this tank!