You don't really have to age your water. Especially if your water treatment system uses chloramine, you have to use dechlor. As far as rain water, I would run it through some carbon before adding to your tank to remove any contaminants, especially if you live in a large metro area or near industry.
Algae is cool to leave in your tank, it can help lower nitrates and gives your fish a good source of plant foodstuffs. If youre lucky, you can get nice green algae and not the black, red or slime yuck that covers and destroys everything (like at my house