Nice information!The most important system in our home is the septic tank. Therefore, it is very important to pump them regularly, as it gets blocked due to accumulated waste products and to increase the life of the septic tanks.
Super old thread here, but you are 100% correct clara, except "tank" should be 'system'.
This was the OP's problem. Provided the OP's system is somewhat functional, their only problem was they waited too long between septic pumpings. Pretty common for people to neglect this. Non-digestible items will build up over time, causing that nice layer of floating nastiness to grow so thick, it plugs the outflow to your sand filter or leach bed.
Dumping large amounts of salt water would be bad for a septic system, just like using large amounts of detergents, soaps, bleach, etc. If it ain't natural and digestible, it shouldn't go down your drain if you're on a septic.
Additives are absolutely not necessary. Many of them can loosen up your floating scum layer to the point it liquifies, flows into your leech field and plugs the whole deal. Newer systems have an effluent filter to catch this (Zabo filter), but regardless, that's still going to plug, have to be dug up to access and clean.
I'm no septic expert, but have replaced systems and lived with them for many years. Good times. For those on municipal sewer systems, you've got it made!