tropical doesnt technically mean freshwater, its just more commonly found to be incorrectly assumed to be freshwater. Tropical shows what part of the world something is from, well self explanatory, tropical fish come from tropical regions, both fresh and saltwater. And please dont ever say freshwater fish are more hardy and/or can live in dirtier conditions, look at the amazon, may be muddy, black (from tannins which is good for them) but that doesnt nessicarily make it "dirty". Freshwater fish actually should be getting more water changes and bigger water changes. The best number is 50% weekly for freshwater fish.
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...Freshwater....Marine