As mentioned you can use any filter on a fresh or salt water aquarium. The only thing to worry about is the filter's turn over rate. Marine tanks have a high pH (8.5) and any ammonia produced in water with this pH is going to be very toxic to the inhabitants. A filter that turns the tank volume over at least 5 (preferably 10) times per hour is recommended to help keep ammonia levels as low as possible.
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Before you invest in a reverse osmosis (R/O) unit, test your tap water for ammonia, nitrite, nitrate, phosphate, pH, GH and KH. If you have clean tap water with no ammonia, nitrite, nitrate or phosphate, then you don't need a R/O unit and can use the tap water (with a dechlorinator) to top up the evaporated water.
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If you don't live near a clean beach, buy some dry marine sand and limestone rocks from a pet shop and use them in the tank. Buy some artificial marine salts too and set the tank up with that.
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Your options for “predators” are about nil for a 40g tank. There are lots of unique, carnivorous or oddball species that would make interesting additions though. Fuzzy dwarf lionfish, hawkfishes, anglers/frogfish/scorpions/stonefish, Valentini and Sharpnose puffers, small filefish species, burrowing jawfish and small basslets all come to mind.
Lionfish, some hawkfish and some basslets can be easy to keep but most of the other fishes listed are not easy to feed.
After covid19 has settled, go visit some pet shops or check out books on marine fish and make a list of the fishes you like. Post the list here and we can make suggestions on what fish will work in your tank.
Most predatory fish can be aggressive to tank mates and many people end up with one fish in a tank because it has killed everything else.