Wow! Freshwater should be a breeze for you then. It should be easier to maintain the water quality with such a large tank, which is a good thing.

I have zero SW experience (well in aquariums). Sounds like it would be good for a FW tank too. Although if you aren't going to have a planted aquarium, the lighting should only be on for maybe 4 hours (or you will have algae blooms).

I always use spring or reverse osmosis water (I bought from the store). The only additive you would need is if you are using tap water you would need a dechlorinator.

You have tons of possibilities with this tank. Have fun. Feel free to ask any other questions you may come across during the process.
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Phishie
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"An ounce of prevention is worth a pound of cure."