My sump really stinks... What's going on???

[bodie]

I've had my tank set up for years, but about 6 months ago I gradually replaced my bio-balls with live rock. The rock was supposedly cured, but just to be on the safe side I cured the rock for a few months before placing it in my sump. When I placed the rock into the sump it had a clean fresh smell to it and I didn't experience any problems over the next few months. But this past week I lifted the cover off of my filter and was knocked back by a gross rotting smell. I checked my tank and all my fish have been accounted for. Everything else seems to be fine and the water in the tank smells fresh and clean. Does anyone know what is causing this nasty funk in my sump?

 

[fishfreek]

Is this live rock submerged in the sump? Is the cover you speak of a tight fitting cover?

If the smell came about after several months I doubt it is from coral or alge die off on the rock. Could be that a large worm that was living in the rock has died and is decomposing.