If you're one of those poor souls who don't "believe in" evolution, this won't help. But there are no rules in fishkeeping other than that fish and plants are diverse, and what works for one, water-wise, may not work for another. There are many fish that have adapted to very hard water, and would thrive in your tanks.
pH is easy to read, but deceptive. It tends to be higher if there are a lot of minerals in the water. Tends. Again, that isn't set in stone (accidental pun there).
It's often a matter of looking at where the fish originates, and then at a map. Coastal species tend to need harder water, as limestone and dead coral are common along coasts. Rainforests have very soft water. I live along the Atlantic coast of Canada, and tested a local lake this morning - very soft and acidic. So we're talking tendencies. In coastal Mexico, relatively recently risen out of the sea, the pH is very high and the water very hard.
We want rules or laws. They just don't work here. Your pH will tend to be what comes from the tap, and almond leaves release tannins and will affect pH in poorly buffered water, but are truly useless except in a cosmetic way. They won't change the minerals, and they are what matter to the fish and plants.
All fish have needs. Guppies still are fragile little creatures and they still need constant and proper care.
I started out with a bettas and killed it in less than a month... Then I did a ton more research and did a fish some on here thought would be too complex for me. These fish have now been thriving in my tank.
The point is, don't settle for a fish. Find one you like. (Of course don't do something like discus cause their pretty expensive and they can be a bit harder to keep alive)...
There are many different types of cichlids and some are pretty hardy while some, its said, are a bit more difficult to keep...
If you go to a serious fish site like .... Seriously Fish, it often gives a quick view of the natural history of the fish. You can also work with the Latin name of the fish and search for habitat, or description. Any good scientific description of a newly identified species should have natural conditions recorded.
If you can locate a specific habitat (a river or lake), sometimes you can find non fish info on the water and weather. It really depends on how much you want to know, and how much that makes you prepared to dig. I try to breed my fish, and information is power in that activity.
I am admittedly jumping into this thread late and have only skimmed the many responses but I'm sure what I'm about to offer has likely already been covered...
Generally speaking, there's no such thing as too much clean, fresh water. After all, with rare exception, nature refreshes fresh water all of the time. So the 50% weekly water change, or more, is perfectly fine. Now this may be difficult for those attempting a water chemistry unlike their tap water. My advice to most hobbyists is to stay in your lane and not attempt to match water unlike what you have. Generally if you can drink it, it can't be bad for fish swimming in it. I know that's a broad statement and not applicable in all cases, but jumping through hoops to match water chemistry can be a disaster for all but very experienced hobbyists.