Just throw them into the toilet bowl and walk away! They are formulated to ensure that odours will be a thing of the past and they even slowly get rid of that pesky iron staining.
You can throw the fizzie into the toilet, let it start it's fizzing and then scoop it around the bowl with a toilet brush to ensure it gets under the rim, simple but it really works well.
18 Fizzies. Septic Safe.