Tank smells powerfully of rotten eggs

[asylumdown]

I'm running bio pellets, it's a recirculating reactor so the pellets tumble regardless of how much flow through there is, but it's possible that the outlet got clogged so not enough water was escaping the reactor. I just cleaned all the screens on it.

I also stupidly did not think of the emergency drain that is underneath my sump. It's in the world's least convenient spot so I don't pour water down it very often. I just topped it off in case the drain trap dried out and it was sewer gas that was escaping in to my cabinet, but that shouldn't have made the water itself smell should it?