I would start by rinsing it all off in hot water. Then tackle any hard water deposits on it with some white vinegar. Just fill the tank with water, dump some vinegar into it, and let it sit for a few hours. Or you can take this opportunity to do your leak test before you get everything nice and clean, and let it just sit full of the vinegar and water for at least 24 hours to make sure it isn't leaking. Then start scrubbing it inside and out using paper towels or a clean new sponge (nothing containing soap or that has been used with soap before). You can soak the filter and heater inside the tank while you let it all sit. If any hard water stains remain, you can scrape them off with a razor blade. Once everything is clean, rinse very well and dry everything off. Good thing about a 5 gallon is you can do it in the tub instead of having to haul it outside.
