I'm sorry but most people who keep brackish or salt water fish in fresh mess up pretty much everything else. Its not salinity or lack there of that kills of most "freshwater" morays. 9/10 times its temp. And so it goes for the unique or special conditions that each and every one of those fish(brackish) has. It goes like this, people say you can keep a salt water fish in fresh except it will only live _ long and _ is usually 3-6 months then when you hit 18 months they are like well you can't keep it for 3 years then when you hit that bench mark they are like well it will die eventually in an aquarium that is fresh. FYI at the 3 year mark most of these fish in question are adult or atleast sexually mature. My mono sebaes are trying to spawn having been raised in fresh... what better testament to success is there than that.
I'm sorry but I've mad that no one recognizes success, when I bought the monos people on here said no you'll kill them then a VERY vetran member on these forums linked me a pick of his FW young adult sebae mono collection.
Simple general reason for why most people kill brackish and salt water fish in fresh is under-filtration. Another one is heat. The typical 72-78 degree tank is too cold for many of these fish that live in extremely warm waters(the whole they stop eating in fresh is usually remedied at 82 degrees). You think a FW ray puts out allot well probably double that ammonia per lbs of the fish. That isn't universal but its a good basis for you to understand one reason most even advanced keepers struggle with them.
I'm not about to say hey everyone can keep these brackish fish in fresh but proper consideration its probably more than possible if the fish occurs in fresh at some point naturally. I'm not saying toss a clown fish in your community tank. That's a horrible idea. But you get where I'm going with this. I don't recommend Brackish in fresh, heck most people even on these forums struggle to properly care for what they have the demands of a brackish fish in fresh are higher for sure but it isn't torture and it isn't impossible when done right.
If you need a more nuanced and itemized approach to how I've kept many different brackish fish in fresh PM me.