Here is the thing. All of the above posts are pretty much on the money but also give a bit of a limited explanation. This is due the great diversity possible in different bodies of fresh water. Seawater tends to have many similar parameters as all the words oceans and seas tend to have connections to some degree.
But in the fresh water universe we have pH 4.0 with almost no hardness. Then we have rift lakes with very hard water and pH upwards of 8.5 in some instances. We have fresh water fishes living in temperatures which would kill most of the species we keep in our tanks.
The result of this diversity is it means it is very difficult to find any true hard and fast rules why apply to all species of fish commonly and uncommonly found in our tanks. I can offer only two things I know to be universal when to comes to FW fish. The first is easy, they all must be kept in water. So is the next, the nitrogen cycle plays a roll in tanks.
As for the rest of the"rules" we may read, hear about or watch on YouTube etc., They all have exceptions. I have dropped the pH in one of my tanks by one full point in under five minutes and the fish were perfectly fine. I learned I could do this from the person from whom I bought my first of these fish. He dropped the pH by 1.1. In both cases there was/is a digital monitor for pH, temp. and conductivity/TDS one the tank. The specific species of fish involved mattered.
Consider salmon. The are born in fresh water, migrate into salt and then return to fresh to spawn. What about brackish? What about fish which live in waters which alternate between a dry and a rainy season every year? The point is, the key to it all for use as ambitious and dedicated fish keepers to acquire the knowledge before we acquire the fish. So knowing where to get good information is half the battle.
When my first tank went up in the end of Jan. 01, there was no social media. There were only forums like this one, only many more. Social media has a lot of plusses, but I also believe it is where information goes to die.
You can start in two place I have used for a couple of decades. For general info and especially helpful with issues re water parameters is.
F I N S : T h e F i s h I n f o r m a t i o n S e r v i c e Aquaria FAQs
When there click on Your First Aquarium and then read the sections under Practical Freshwater Chemistry
. Next, for decent information if fish species in general always start here and if they do not cover the specific species you are investigating, try elsewhere. I small amount of the info there is a bit off or incomplete. I do not agree that the nitrate test is essential- for me it is pH, ammonia and nitrite. Then KH and GH and finally nitrate if you can afford them.
I find the fastest way to get to the species page on the above site is to start on Google search and then type in either the scientific name or the common name of the species followed by the word seriously. Most time it takes you to the exact page. Some common names are shared by multiple species and this may or may not mean you find exactly for what you thought you were searching.
Also, always listen to Byron
, he and I almost never disagree.
edited for typos