Im also just using prime and tap water. Works for me :)

Good point, but still I see no point in starting with water that is anything more than 0 tds. You know what you're putting in the tank that way.

When I first started was using tapwater and found the issue was more cyano with a little hair algae. After about a year switched to bottled ro/di and there was a definite and continued improvement. Now how much was due the new tank, new hobbyist?

Nothing wrong with trying the City's and if having problems switching to RO.

Seems from the post your talking bottleb City or bottled RO, or are you considering your own ro unit? If you talking a smaller tank bottles are doable but if a larger tank if you can make your water on site, of course way more convenient.

I have a question what is RO/DI slang for? I want to start a SW aquarium and i need to know if i should use Tap water or if i should use distilled water.

Go
 

Reverse Osmosis with De-Ionization (RO/DI).

I used Edmonton tapwater for years in fish only tanks. I have a 6 stage RO with DI that I now use for corals.
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homemade vaporizer

Well i want corals so would i be able to buy some Distilled water or for a RO/DI system how much would it cost for one for a 20g tank?

Go
 

You can buy RO at any grocery store or at Home Depot. It's about a dollar a gallon.
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Honda XBR500

So i should buy like 50g before hand when i start for water changes and major spikes in the parameters.
Thanks!
SeaSerpant