Is cycling necessary for fish---no! But!---

[Sk8r]

There's a lot of magic-thinking involved in this topic. Do fish NEED cycled water to live? No. Do they need a cycled tank to live without fish poo doing nasty things to the water? Keep reading.

Let me be clear: fish will live in squeaky clean bacterialess salt water just fine. KEEPING the water that clean is labor-intensive, involving pulling filter medium every time you spot a slight stain on it and KEEPING the tank from cycling while fish are in it. Under heavy pressure, this can be more than once a day. But live? Yes. The fish and even corals will be ok.

How can this be?

There is nothing unbreathable about clean salt water if you're a fish. What you don't want is ammonia. Ammonia WILL result from bacterial action on fish poo and other waste. As long as you keep that filter scrupulously clean, a cycle (ammonia) won't happen and the fish will be fine.

So why do we cycle tanks? So that microlife will live, so that you will have a fullblown ecosystem producing little crustaceans and multiple sorts of bacteria that first break down poo to ammonia, to nitrite, then nitrate; the bacteria will take care of the ammonia before it hurts anything.

What a live-rock tank does is a multistep process. Nitrogen gas is the final product of a cycled tank with live sand and live rock, so that nitrate stays low and ammonia doesn't have a chance.

If you had only a bioactive fiber filter, that filter cannot sub for live rock and a sandbed: the nitrate would ultimately rise quite high, too high for fish to tolerate, if that fiber filter were not cleaned out frequently.

But will your fish, in qt or in an emergency, live in plain salt water just fine, granted you do the work to keep that tank from starting a cycle? Yes. If it's a temporary situation and not the ultimate destination for the fish, you can keep changing filter floss (I recommend pillow floss from the fabric store) whenever it stains slightly. That guarantees, so long as you keep it that clean, that it will never cycle and never go toxic with ammonia.

Also: in an emergency, though we stress always using ro/di water---keep a bottle of Prime or other water conditioner on hand, along with a LOT of salt, and if you need to create a qt or hospital tank on the fly, just condition tapwater with the Prime, add salt, mix with a strong pump, and as soon as that water is clear and not cloudy, it is ok for fish or anything else you need to shelter.

This is particularly valuable to know if you ever face a tank crash: think about it: you want to lighten the bioload on the system that is on the edge of a chemical crash---so get all living things out and don't let anything die in there. (A dead mass creates even more gunk, which is what you want to avoid.) This includes corals and inverts. Get them all to clean water in a clean glass box, and then hope your tank bacteria can handle the situation and pull the living system back from the brink.

Nothing much magical about cycling---except its ability to do that particular magic and turn poo into nitrogen gas, which floats up to join the Earth's atmosphere and cleans it out of your tank. That, with your skimmer (which removes protein waste), produces a small 'circle of life' in your tank.

But clean water is always a good refuge.

[xanthurus]

I'm ok and agree with this part.

Originally Posted by Sk8r

How can this be?

There is nothing unbreathable about clean salt water if you're a fish. What you don't want is ammonia. Ammonia WILL result from bacterial action on fish poo and other waste. As long as you keep that filter scrupulously clean, a cycle (ammonia) won't happen and the fish will be fine.

I am confused by your description of the nitrification cycle here. It doesn't seem to match what is talked about here, http://www.reefkeeping.com/issues/2002-06/dw/index.php. I thought the process goes from waste to ammonia to nitrite to nitrate to nitrogen gas. Could you please explain.

Originally Posted by Sk8r

So why do we cycle tanks? So that microlife will live, so that you will have a fullblown ecosystem producing little crustaceans and multiple sorts of bacteria that first break down poo to nitrate, then nitrate to ammonia, and then take care of the ammonia before it hurts anything.

What a live-rock tank does is a multistep process. Food to nitrate---Nitrate to ammonia: nitrogen gas is the final product of a cycled tank with live sand and live rock, so that nitrate stays low and ammonia doesn't have a chance.

If you had only a bioactive fiber filter, that filter cannot sub for live rock and a sandbed: the nitrate would ultimately rise quite high, too high for fish to tolerate, if that fiber filter were not cleaned out frequently.

[aujosh84]

Yup. Gonna chime in on that too. Thought it was poo -> ammonia -> nitrite -> nitrate -> nitrogen.

25,863 posts of awesomeness, but you make 1 mistake and get called out by everyone. :-)

[Patchmty]

Caught that too. Looks like you inverted ammonia and nitrate, and skipped nitrite

[Sk8r]

Yep. You're right. Comes of going too fast. But the point remains. One of those days. I'll fix that above, but leave the thread intact, because you're right to call me on it. I know that, in any other compartment of my addled brain. Poo to ammonia. Ammonia to nitrite. To nitrate. To nitrogen gas, if you have a full breakdown.

[Raul-7]