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Welcome to AA. Hopefully someone who's been through this before will chime in. I have no experience, but would recommend vinegar and water, 1:20, and a lot of rinsing.
I'm not a saltwater keeper but in my very humble opinion I believe you just need to wash most everything with hot water to get all the build up off, and put in new filter media.I say most because I would not use any of the crushed coral or sand form the previous tank, those things have a tendency to keep the Ph up on the high end. And now since you are going freshwater you will not need any of those fancy bulbs needed for a SW tank. Trade up or sell your old set ... think of the money you can save. - hope this helps
A couple really good rinse and drys should be all you need unless you have coraline algae build up which a razor scraper will do the trick, but be careful in the corners to not cut the silicone. use a tooth brush or other similar nylon bristle scrubby brush to get any off the silicone. this is of course assuming it's a glass tank.