Can fish live in uncycled water? FYI

[Sk8r]

Absolutely. There's absolutely nothing about cycling the tank that makes the water better or worse for fish or corals or inverts. So if your tank water is for some reason rapidly going south of 'good', uncycled new salt water is the best safe refuge for your critters.

So what is cycled water?
It's simply water from a tank with a lot of bacteria that can process fish poo efficiently into nitrogen gas bubbles. That's it. That's all.
Is it ok for fish to live in bacteria laden water?
Doesn't bother them. Doesn't particularly help them.
What DOES bother fish is the nasty trick fish poo does when it clogs up in filters and makes nitrate in the water. A heavy load of nitrate in the water is NOT good for fish, corals, or inverts---and it has the real nasty habit of producing ammonia, which is lethal to fish real fast. [You'll notice I didn't say 'corals and inverts' in the same breath as fish, there. That's because some corals can survive ammonia which would kill fish: but don't make them have to!]

So if your water has for some reason gone 'dangerously awful,' the very first action you should take is getting your fish to clean salt water which, no, does NOT have to be cycled. What it has to be is fully dissolved, because undissolved granules of salt can burn gills. To this end, use a potent mixing pump in your saltwater mixing barrel. I can't recommend how strong because I don't know how big your barrel is, but say that the water should be roiled up and moving, and the more so---helps. Mix until that water is crystal clear, no cloudiness. Then it is safe.

Changing out part (say 30%) of the water in your tank for new WILL hammer down the nitrates and pull a tank back from the brink.
If your tank is headed OVER the brink and fish are in distress, just get that new water warmed and get them into it.

Must you use ro/di in an emergency? NO. Always have a bottle of Prime at hand. This takes care of the chlorine issue. You can also heat cups of water in the microwave to bring the water temperature of the whole up to level before you add the Prime and prepare it for adding salt. Measure! The ratio is usually one precise half-cup of salt to every precise gallon of water to yield 1 gallon of 1.024 saltwater. As soon as this water is mixed clear as crystal, it is useable.
There will be some consequences for using this emergency water in your tank (as non ro/di) but those involve only a little algae bloom, nothing so serious as a full blown tank crash. So can you use non/rodi warmed in a microwave to solve a crisis, once brought to 1.024? Abso-tootin'-lutely.

As water for fish or corals, absolutely, again, use it if you have to. The little amount of phosphate it may bring in is NOTHING, if it saves your fish.

And if you have to prioritize in an emergency, get the fish moved first...the corals can protect themselves by contracting and expelling water, and again, don't like to suck in undissolved salt.

As a note, take nothing in these tanks as a Mystery, or a We Just Do It This Way. Always ask why. You need to understand WHY we cycle a tank and what it does in order to make intelligent choices if confronted with a problem.