The truth is, saltwater aquarium fish have far different pH and water quality requirements than their freshwater cousins. I would guess when you kept freshwater fish you didn’t pay much attention to water parameters such as calcium and alkalinity; though in the saltwater world these play an important role, even in fish-only systems. live in very stable conditions naturally. Because most of the fish that we keep in our saltwater aquariums are captured from the wild, we must be willing to replicate these conditions in captivity. This means providing the aquarium with substrate that can buffer water pH, alkalinity and calcium in a manner that produces minor fluctuations. On the saltwater side of the hobby we use aragonite sand, crushed coral and live rock to achieve this. These substrates (in the case of the sand and crushed coral), are very dusty, though this dust helps the water buffer to create the right parameters. Most freshwater substrates are chemically inert therefore they don’t provide any buffering capabilities. So at the least you would need to replace your aquarium gravel with either crushed coral or aragonite sand.
Saltwater Fish, Reef Safe Fish, Aquarium Fish - Blue Zoo Aquatics
I have a 72-gallon bowfront fish aquarium with a mixed community of freshwater fish. My daughter loves the color and vibrant nature of saltwater fish and I am thinking of converting over. Is it difficult to convert a freshwater fish aquarium to saltwater? Can I just remove the fish and add a sea salt mix to my existing water?
Beginner Fish: Tropical Fish for Beginners in Saltwater Aquariums
An aquarium salt treatment can also be used regularly to reduce goldfish stress and stimulate slime coat production. Plus, salt can kill off parasites harmful to goldfish, nipping one problem in the bud!
Saltwater Fish Aquariums & Habitat - The Spruce