Here’s where the nerds hang out. Reverse osmosis (RO) and deionized (DI) water is the holy grail for serious fish keepers. It strips out everything—chlorine, heavy metals, even unicorn tears.
But you need a special filter unit, and they’re pricey. You also have to remineralize the water afterward. Yeah, it’s a whole extra step. Worth it for a $200 saltwater setup? Absolutely. For your goldfish? Probably overkill.
Unless you’re breeding something super fussy, like crystal shrimp or Amazonian tetras, skip RO/DI for now. You’ve got a life, right?
Rainwater: Free, but Risky
“Hey, why not collect rainwater like our ancestors?” Because your roof is covered in bird poop and asphalt particles, that’s why. Rain is acidic and full of airborne pollutants like car exhaust and factory smoke.
What Water Is Used For Fish Tank at Ryan Moorman blog
Unless you live on a pristine mountaintop, don’t do it. Seriously, your fish did not sign up for a chemistry experiment involving acid rain. They want stability, not adventure.
