I really don’t intend to argue with Leon above, but as a seasoned aquarist, with the experience in saltwater brackish, discuss the various types of freshwater fish. I will tell you that you can definitely take your freshwater aquarium and totally turn. It brackish with only specific aquarium inhabitants in side. For example, mollies are able to adjust to freshwater as well as salt water. If you research on other types of fish that are able to go between freshwater and saltwater, you will see that there are others as well. The advantage to taking your fresh water through the quasi brackish to brackish to full on salt water status is that you bring your salt water aquarium through fully cycled You do this slowly or at least I did over I want to say at least weeks. Gradually increasing the amount of salt with each water change. And once again, this is only with specific fish that are able to handle this do your research.! Certain invertebrates such as certain snails and shrimp may be able to go back and forth do your research. This is not intended to be an exhaustive article. I just wanted to tell you there is an option and the whole issue of scrub area every Drop of freshwater out of your aquarium before transitioning to saltwater. No offense is really pure bunk. I have transitioned more tanks than I can count from fresh water to salt water with babies with broods without losing any of them. What is worth noting however, is that going from saltwater to freshwater will not work! For some reason, my experience shows that the bacteria or whatever Live organisms are in the salt water when you go from saltwater to freshwater, it corrodes or corrupts the objects (like decor and filter media) in the water and it becomes unusable —it develops what appears to be some thing similar to white, fuzzy, mold or I don’t know what you wanna call it but it’s not good and it’s not nice and I do not recommend it .
With regards to copper : is actually included in many fish foods and even some shrimp foods, so that line of reasoning that: “ a tiny little infinitesimal scrap of copper from decades ago is going to destroy your whole aquarium. “ That’s BS. I’m sorry. Not trying to be argumentative, just honest. To be fair, you do want to ensure that you do not have any free-floating copper quantities in any aquarium that harbors invertebrates that you value in other words that you don’t want dead. And of course, copper is used to destroy parasites and diseases on fishes of all types but of course we don’t recommend to have them swimming in full Copper solutions. And if you think you may want coral, well, reef tanks are very particular, especially SPS coral. Etc. I would definitely avoid copper in any tank that I intend for coral. Nevertheless, read up on some of the “dips “ refers use prior to adding any new corals to their existing tank.
Also, note the freshwater dips Reefers use to treat or prevent disease in the saltwater fish as well as the saltwater dips freshwater hobbyists used to treat their fish for disease or to prevent the same. I personally prefer salt, water, and freshwater dips as opposed to medication‘s because medication‘s have killed more of my fish than natural causes, and even bully fishes. Dips are one of the safest and most effective ways to treat and prevent disease among fish.