30min isn't long enough to cause problems but it's probably related some how. Are you certain power was only out for that long? Does your equipment start up properly after power outages?

Tank MAY not be accurate to test as dead animals pollute a tank fast.

Check temperature, aeration and salinity.

I would also change some water if you have it ready, if not get it ready asap. Check flow it sounds like low o2 levels

I as far as i know, I was at home when it happen..

did a water change and i have placed a wave generator near the surface to create air bubbles...but now I'm screwed because its after 10 am here and i'm supposed to be at work...fml

did a water change but now i'm screwed i need to go to work now fml i placed a wave generator near the surface to created air bubbles to add o2 to the water. the remaining fish seem to have a little bit more energy and colour.. I'll let you guys know how it goes

good luck :) Hope it works out..

Sorry for your loss though :( no good

Low oxygen. That would be a guess, except for the knowledge of power failures which make that guess an almost certainty.

When you get running, you have to simulate that power outage again. Watch what happens when you turn the power back on. Do ALL the pumps come back on? Does the skimmer come back on? Do you have a skimmer? If not, this episode suggests that you should because they will add O2 to the water, especially after a power outage.

The other thing you might need is a battery operated pump that will bubble or move water during power outs.

heh
 

Take a sample of your water into the fish store, they may charge you $10.00. Atleast you will know everything.

Probably too late now, but we don't charge anything to test your water, if you ever need a second opinion, feel free to come in.

Related to your post though, I would bet money on a O2 shortage, do you have a sump? If not, this is probably the cause, tanks without sumps can run fine under optimal conditions for years, but all it takes is that momentary lapse in stability to throw it off..... As others have stated, try and simulate the event and see what happens.

Steve