What kind of water should I use?
There's a lot of talk online about the right kind of water to use when watering your plants! There are a surprising number of types of water, in fact, each with different qualities, upsides, and downsides.
Tap Water
Upsides: Obviously, super easy to get and generally the cheapest option. Generally a good option!
Downsides: Tap water is generally chlorinated to make it safe for human consumption, which can in some cases cause damage to specific sensitive plants, and only sometimes. I can't stress enough that this is a rare occurrence, and most of the time when people think that it's the culprit, it's actually low humidity.
Note: You used to be able to just leave water out to "off-gas" the chlorine -- and you still can... maybe, depending on where you live. Most places now use chloramine instead of chlorine, which doesn't off-gas. If you're worried, use a dechlorinator made for fish tanks!
Rainwater
Upsides: It's as nature intended! Generally free of harmful chemicals and heavy minerals. Plus, depending on where you are and how you collect it, it's free!
Downsides: It's just really annoying to collect, honestly, and in my opinion flat-out not worth it. Timing is erratic and uncontrollable, it's hard to guarantee you'll have the amount you need... so much is out of your control.
Distilled/Reverse Osmosis Water
Upsides: These are very similar types of water -- both are by nature very pure, free of any pollutants.
Downsides: Plants actually do get some necessary nutrients from water! Watering entirely with distilled water will require that you re-mineralize it occasionally, or use a comprehensive fertilizer with micro-nutrients.
Note: Only ever use distilled/RO water to water carnivorous plants! They're very sensitive to nutrients and minerals, because of how they evolved, so this is one case where you want only distilled/RO!
Fish Tank Water
Upsides: Honestly, probably the best option overall! Nitrates (the reason you need to do regular water changes) are a perfect source of nitrogen, fish waste provides all sorts of other nutrients, and if you're fertilizing your fish tank plants, some of that will still be in the water as well -- plus, it's dechlorinated and heavy metals are removed with the water conditioner, so no need to worry about sensitive plants. You're also re-using water, which is always a plus.
Plus, fishy friends!
Downsides: You need to keep up at least one fish tank, and if you have a large collection, you may need either a fairly large one, or multiple tanks -- and that's a whole other world to explore! Plus, to get the water to water your plants, you need to do a water change, then collect that water so you have it on hand; it's a decent amount of extra work.
(But.. fishy friends!)
Overall Winner:
Definitely fish tank water, in my mind (and in my home -- I have three tanks to provide the water)! It's naturally-fertilized, so you don't need to add extra fertilizer to your water -- and sensitive plants are fine with it!