Oh boy. THAT's what I was expecting! It was rank, and foul, and eye watering.
We quickly duct taped a plastic bag over the access hole to keep the evil in, and turned on the ozone generator.
Five minutes, it took. Five minutes to almost totally kill the smell in the water. It still looked foul, but it only had a faint organic and ozone smell.
Ozone is a strong oxidizer that breaks down organic matter on contact. It's like burning things, only in water. So we left it running all day. After 6 hours - and several duct tape changes because the ozone was killing the tape - the tank water was black with carbon and quite hot. The heat is a bit subjective since the unit is inside a tin shed and it was 41C outside. I think the lawn fertiliser and lurking horrors have been broken down into basic molecules.
Take that, you coliforms! I have defeated you with SCIENCE! MWAHAHAHAHAHAAAAA!