Mostly my experiences,
For normal tap water.
It will restrict your fish stocking capabilities. Some fish don't do well here or there. Some fish do well anywhere. Tap water has lots of unknown elements in it. Phone or e-mail your water company for a test sheet of your supply.
Perhaps a useful indicator of co2. PH Kh and co2 are closely related.
I think it's written, soft fish can adapt but not all soft fish, some hard fish can adapt to soft, but not all fish. It's easier for soft to adapt to hard conditions because they need less minerals naturally so the abundant water is easier on them, hard water fish naturally need more mineral uptake from the water and in soft water those minerals are simply not apparent. Make of that what you will.
That's a big question, ammonia and nitrite are affected by pH, they become more or less toxic depending on pH.
I think hardness is more important than pH but pH shouldn't be dismissed, it is useful to know as it leads to a lot of other answers. For each test you perform you can find a valid reason for testing.
Loads of things are important, temp, GH/Kh, pH, nitrogenous waste, TDS. Those are your freshwater basics.
Add iron maybe if you have a planted tank, or a massive water change.
Kh/pH calcium magnesium temp salinity and nitrogenous waste for your reefs. Or so I believe at this point, almost an owner but not yet. Maybe phosphate and par?
There may be more or less, I may even be wrong, currently that's what I think.
Others may have a different approach and set of tests that also work.
Some don't test at all and have no problems. (That's my disclaimer
)
Lots of readily available "common" fish will adapt to most conditions, while they may not be optimal they will do. Fish live in water. I think it's best to try and match and maintain ideal conditions. The fish look better and behave different. They are a good indicator of conditions if you let things slip.
Lots of fresh water often and the same will see most fish alive for the given lifespan of the species. You will also find line bred farmed fish are much more forgiving than a random wild species. The behaviour of wild vs tank breds is sometimes very obvious.
Aquarium Nitrogen Cycle | Cycling Methods | Ammonia & Nitrates
That's a pretty good read. Most places you can read about the different things, I'm not typing all that!
