H2S in stagnant standing pipe water

[Delphinus]

I've had my sump shut off for a couple of weeks. Kept the water in the sump circulating with a powerhead and a heater. Had the water level in the main tank artificially raised beyond the intake to the overflow by having the tank's downspout valve turned off.

So I opened that valve, and let the main tank drain to the top of the standing pipe into the sump.

The problem is the water that was sitting in the standing pipe all this time. It's most foul, the smell of rotting eggs came wafting out of the sump after that water drained into it.

So, I'm thinking, I just created myself a 20 gallon water change. Is this correct? Ie., I assume this water in the sump is no good now? I should just dump it, and start up a new batch before turning my sump back on? I assume the smell is H2S...