Well - kind of depends on what kind of SW tank your looking to have.
So you have a 65 gallon tank - that cuts out the possibility of any shark or ray(tanks way to small for their lifetime).
But you do have a few good options.
Fish only - where you have your choice of dozen of species that can live quite well in a 65. With some possible cool species like smaller species of eels, tangs, dwarf angels, dwarf lions, triggers(smaller species), puffers(smaller species), & so forth.
Invert only - crabs, clams, shrimp, small lobsters, snails, urchins, & starfish.
Reef tank - coral and/or live rock with smaller fish and inverts.
As for requirements.
Well you already have the tank.
But you'll need the following
- a good sand or crushed coral. -
- wet/dry sump - rated for at least 75 gallons. -about 100-150 bucks
- a protein skimmer - also rated for 75 gallons. - also figure about 150.
- a good pump - to move the water thru the sump & skimmer(capable of 300gph).
- a good brand of sea salt (get the one for 100-150 gallons) - general runs about 40-45 bucks
Of course - you might be able to save a bit on the sump & skimmer - if you either DIY or get them together.