Hi Alan,
We have had a septic tank since 1983. They work on bacteria, so any kind of bleach is bad, as you say it kills the bacteria.
When we had ours fitted the engineer told me if I had to use bleach to keep it to a minimum
and only use it maybe once every two weeks. I do use household cleaners, but I try not
to use bleach too much, I usually use the bleach that's cheapest in the supermarket at the time, but I took the Engineers advise and only use it every 2 to 3 weeks and for the last 26 years it's been fine.
Incidentally, he told us when we first installed the tank to get a large piece of rotten meat, like a leg of lamb, throw it into the tank and it would start the bacteria working immediately....
Jacqui.