Saltwater fish in Freshwater diluted with salt

[CGandC]

Suppose I dilute a freshwater tank with an appropriate amount of salt, will saltwater fish be able to live in it? if not, then why?

 

[BillTre]

Most fish are tolerant of only a limited variation in the saltiness of the water in which they reside.

However, different fish have different abilities to go from salt to freshwater or the other way around.
Some (like salmon) change their abilities to do this based on their stage of life. Salmon start in freshwater (as eggs), then go to the ocean, then return to freshwater to mate and lay eggs. Their physiology changes to make this possible.
Bull sharks are usually found in oceans but can go far up rivers.

Some fish have benefits from more or less salt. Slightly salty water is good for some larval fish at certain ages. It takes energy to pump excess salt out of their bodies, so adding a little salt (but not too much) to the water can benefit them.

Generally speaking, brackish fish (fish in water where fresh mixes with salt) probably have the largest tolerance. (there are some really interesting brackish fish to keep if you are a fish keeper, but as said above, they are more difficult.)

The major take home lesson would be to research what the specific kind of fish you are interested in can tolerate at what time in their life cycle.
There are many resources for this information: books, magazines, websites, extension services, aquaculture organization, hobbyist organizations.

 

[CGandC]

Well, considering a standard saltwater creature that only lives in the ocean ( not living in brackish waters, not diverting between salt and fresh waters, etc. ) - a clownfish for example. Assuming my aquarium with salt ( at the "right" concentration for the clownfish to still be considered hyposmotic relative to tank's water ) is fully cycled, is it suffice for the clownfish to live to its full lifespan?
What parameters have to be "added" to a freshwater system in order for the clownfish to live its full life span?

 

[BillTre]

A quick google search: "clownfish freshwater tolerance" shows a wide consensus that clownfish can not tolerate freshwater.
Here is one result:

Ocellaris clownfish prefer a water temperature between 72-78° F with a pH between 8.0-8.4. As saltwater fish, they require a stable salinity between 1.020-1.026. Clownfish cannot tolerate any traces of ammonia or nitrite.Jan 31, 2023

They are however easy to keep in salt tanks.

 

[CGandC]

Thanks! this clarified things for me. I was perplexed as to the basics of the chemistry that sustains the life of saltwater fish from reading about discussions of what it takes to establish a saltwater tank: powerheads, reverse osmosis system, protein skimmer, etc. Of course, these are important to the keep the fish healthy and etc but not the ground basics of why they are able to live in saltwater.

 

[.Scott]

As a child my parents had friends in New Jersey. On a visit to those friends, I was playing on a beach when I accidently captured a small fish (about 1 inch long) in a sand pool I had dug out. Those friends kept my prize in a small fish bowl with tap water - and a few days later report by phone on its progress. It had slowly inflated to many times its volume before dying.