Sounds like it's time for a pump-out if your odor is in your backyard; if you have odors only in individual sink or tub traps, they may just need a snaking or a cleaning . Bleach isn't good for septics--you can dump 1/2 a box of baking soda to 1/2 a small bottle of white vinegar for a gentler way of doing each drain.
If the odor is coming from every drain, though--you probably need a pumping.
Another one of the ways to determine if your septic system needs a pro to come out and pump the tank also is that your toilet flushes more slowly than normal.
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