How long can Plants like Caulerpa Survive in a Refuguim without sunlight ?

Long enough for some of the slime to die off, but not long enough to wipe it completely out. And it will just return. Your PO4 is showing near 0 because of the red slime (and to a lesser extent the caulerpa). Without the nesiance slime algae, your tank inhabitants would be exposed to elevated PO4, not a good thing. I would start tracing to see where any possible PO4 is being added to your tank; top-off water, thawed fish food water added during feeding, etc. Start cutting back the lighting period on fuge. Down to 10, then 8, then 6. Replace the fuge light if its older.

Aquaknight gave you some excellent advice.

To answer your question in terms of how long, 3 days max, depending on the species. Keep in mind that when the light period for caulerpa is altered (especially when shortened), it runs a higher risk of going sexual. I do not like running the light on caulerpa for anything less than 12 hours, so be very careful about cutting back the time.
Even if you kill the red slime, it's going to come back if you don't find the cause, so that's where I would focus the most concern.

hey question....sry.....when chalupra goes sexual does that mean it spawns spores? I think ive got some growing in my DT!

Yes, but it also releases a ton of CO2 and can crash a tank. If it went sexual on you, especially in your size tank 1guy, you'd know it immediately.

You probably had some pieces just get loose and make their way into the main tank.

AK and Tommie Lynn made good points.

 

IME, just because Caulerpa goes sexual doesn't mean your tank will crash.  We let one of our setups go south on purpose "just to see what happens", and yes, it turned the water cloudy, but nothing in the tank died (this was a small tank).  A WC and some carbon made it good as new.  What you DO need to consider is if you have a healthy amount of the stuff, and all of a sudden it's gone, you've lost a HUGE part of your biofiltration, and any algae issues will be magnified (this also covers manual removal of large growths).

 

The reason macro goes sexual is a last ditch effort to survive when conditions become severe, such as abrupt temp changes, big swings in water chemistry, over-pruning, being thrown into the dark suddenly (most do OK as long as the tank gets ambient light, but if you mask the tank off for any appreciable amount of time, you could end up with a problem).  The macro becomes whitish/and will typically have dark green spots on it (the gametes).  If left unchecked, the macro dies and releases the gametes into the water to colonize elsewhere.

 

Hopefully, the links for these pics will work:

 

Nuked Tank (it was an 18T):

 

http://i285.photobucket.com/albums/ll73/hixphotobucket/macros/opps.jpg

 

 

C. racemosa...normal vs. "about to blow":

 

http://i285.photobucket.com/albums/ll73/hixphotobucket/macros/macrosexual.jpg

 

Yes, almost any pellet and flake food is loaded with phosphate. They're also usually not very nutritious for your fish. Stop feeding pellet, and add a phosphate remover should help.

+1 no flake nor pellets....something i had to learn!

All fish food is loaded with Phosphate.  It's a major element of any nutritious food. 

 

The issue with flaked food is that a LOT of nutrition is leeched into the water long before any fish have a chance to eat it.  Feed something meaty like silversides or Mysis Shrimp.  Something that doesn't dissolve in water.

 

It does help certain algae grow, but too much iron can cause A LOT of issues. Are you dosing iron?

Also, I seem to remember that you had a lot of fish in your tank, right?

If you're adding Fe, are you testing for Fe?

 

One of the cardinal rules of the hobby is "Don't dose anything you don't test for."

 

Also, to go a step further on the frozen foods:  Be sure to rinse them under the tap before feeding them out.  The "juice" is full of all kinds of things best left out of your tank.

i dont rinse in tap but put it in a cup with my RO....pour it through the net and than rinse it again from the net back into the cup with RO....just my .02

Yes

They're not going to remain healthy forever, and you're truly doing your fish a injustice by keeping all of them in there. It's obvious that you're having water quality issues if you have algae issues such as these, it's going to end in disaster.