Yes using riddex in your septic tank really does work to help break down the poop and waste in the septic tank so that you don't have to worry about filling up the septic tank so soon and having a possible sewage backup.
When I lived in a house with a septic tank I would dump a box of riddex in the toilet and flush it down every few months and it helped to break down the waste.
I didn't have to have the septic tank pumped as often as I did without using the riddex so in my opinion riddex really does work to help keep a septic tank clean and prevent it from filling up too much.
You still need to have your septic tank pumped though which you should do every 5 or 10 years depending on how much use the septic tank gets.
With my septic tank I had the septic tank pumped every 5 years and sometimes it didn't need it but I had it done anyway as it cost me only $60.00 to have my septic tank pumped.
The cost to pump a septic tank can vary from $50.00 to $150.00 depending on where you're at and how large your septic tank is.