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Saltwater fish are more sensitive to changes in their environment.
Saltwater fish are more tolerant of higher nitrates than invertebrates (in general), but still like nitrates lower than 20ppm, with less than 5ppm being required for most invertebrates.
Saltwater nitrifying bacteria are different than freshwater nitrifying bacteria, so they must be cultured from scratch.
The cost of a saltwater fish and aquarium can be cost prohibitive because the fish are more expensive than freshwater fish and can require lots of upkeep on water quality and health of the saltwater fish.
Saltwater aquariums should be no smaller that 50 gallons and can range up to 200 gallons or more.
Saltwater fish require lots of places to hide and dart in and out of to get away from more aggressive fish.