Ya, if you've got other forms of oxygenation like surface agitation or an overflow into a sump a skimmer flaking out shouldn't nuke your tank so quick. However, if you don't have any of the above and the protein skimmer is the only thing oxygenating your tank then you are in trouble if it goes. If the tank is relying on the skimmer like this and the skimmer goes your fish can and will likely die.

another Casualty
 

Coral Banded shrimp is now Nem treat

Whats your ammonia at right now?

I stop my skimmer each night..never killed anything.

I really doubt this has to do with your skimmer stopping for one night. I have been stopping my skimmer for the night since the tank was started a year ago. I stop my skimmer from 10:00 pm till around 9:00 am, but when I left for vacations the skimmer was off for a week. Nah...something else is going on in your tank. You should verify for disease, if you added something recently. Velvet is not always apparent. Also you should check that your powerheads are doing enough surface agitation.

Right now in my nano 21 gallons I have a 3" blue hippo (sold and going away friday) a 4" kole tang (temporary there) and a blue damsel, all without skimmer. Everybody are top shape. I run the skimmer once in a while, maybe once every 2 days. Neither of my tank have a sump. Simply water agitation with circulation pumps.

Dude crappy deal! I agree with everyone there has to be another underlying problem to kill that fast. Check your parameters carefully and let us know what they are.

It certainly is worth investigating other potential causes. Although, I do find it suspect that this happened right when your skimmer died. Were your fish showing any signs of distress? Rubbing against rocks? Have you tested your water parameters as Grizz suggested? Some fish not eating?

Still, in my opinion, this is a high bio load to sustain without proper oxygenation for a period of time, especially with the only two powerheads not pointing at the surface for surface agitation. It doesn't take long for one or two fish to get stressed due to lower oxygen levels as some require more than others. Then they start gasping and breathing heavier and heavier depleting oxygen faster and faster. You multiply this by a few more fish and you could be in big trouble fast. Then your first few fish die and you pretty much have a meltdown :cry:

http://dl.dropbox.com/u/18798128/36%...%20001-800.jpg

What is the temp of the tank, the higher the temp the lower the oxygen capacity of the water is and the wasted the metabolic rate of the fish is.
I do not think the death is from the skimmer failure, I have 7 fish in a 33gallon tank that has no overflow, no sump, no skimmer and I have no problems. I have an mp10 turned way down and a koralia that's it that's all. With weekly water changes everything is awesome.
How often do you do water changes? Do you have a lid? What about salinity swings due to evaporation?

Consider also that the fish are not the only things consuming oxygen in your tank. Nitrifying bacteria and algaes will consume oxygen as well. Further, at night (when this skimmer died incidentally), some algaes will breath and consume oxygen contributing to the oxygen depletion.

Also, oxygen saturation will occur somewhere between roughly 6 and 8 parts per million in a saltwater tank while this number ultimately depends upon temperature, salinity and elevation. Temperature has the most effect, higher temp means lower oxygen holding potential of the water as outacontrol mentioned. This means that the water molecules have absorbed as much oxygen as they can easily hold under the present conditions. Any more oxygen added will be released back into the atmosphere.

Ultimately you have to get oxygen into the tank somehow. It doesn't have to be a skimmer, or an air stone, or a powerhead, but something has to add oxygen to the water. If the tank relied on one source of oxygen (whatever that source was) and that source happened to go and your tank was now consuming more oxygen than what is being added, you can reach a point of deprivation.

Occam's Razor.

What are your water parameters?

Temperature 79
Specific Gravity 1.025 1.024
pH 7.9 8
Alkalinity 220 mg/l 196.9/1
Ammonia 0 0
Nitrite 0.1 0.01
Nitrate 20 20
Phosphate .5 .5

Last 2 reading prior
will update
Appreciate all the help.