I have 3 toilets on septic system.

1 upstairs -older model (higher flow) clogs frequently, occasionally overflows. Water
Gets fairly high to lip even when unclogged, is that normal?

2 downstairs - newer 1.6 models. 1 needs plunging frequently, other not so bad but occasionally. Neither tends to overflow.

What's the best way to diagnose/fix? Get the septic tank pumped? Snake? Something called a drain king? Since its more than 1 toilet I figure it can't be a toilet specific problem.

If your water comes from a well it is possible that you have a high content of different dissolved rock in your water. When water sets in a commode for hours some of this rock precipitates out of the water and sticks to the porcelain and make it rough therefore creating friction when the commode is flushed, tis will make the commode flush slowly and the water will swirl around and around and not go out as fast as it should. The something will happen if you urinate in the commode and not flush it, the salts in the urine will forme ont the finish also. If this is the problem you need to use some type of cleanser that is save for a china commode.

Dudley4me- I would say that you more then likley have a full septic tank. Check to see if you have a clean out between the house and the tank,If you do open it up and see if you have water standing inside the sewer line, if you have a vent comeing up next to the house as some do ,you should be able to pull it up out of the "t" in the main sewer line and check for standing water in mainsewer line. If you can while watching have someone in the house to flush one of toilets you have trouble with,see if water comes out but backs up from tank, the other way to check is dig up tank to where the hole is for emptying the tank out, pull the lid up out of the tank ,to see where the wste level is at in side tank. This is some ways to do some checks to see if the problem is the tank or the sewer system under & inside house. But from all my exper's. from what you describe , I'D SAY the tank is full. Good luck,&have a good day,GOD BLESS.::::: F.B.E.

Well you are not going to like my suggestion, stick you finger in it. Yep, you need to inspect the small hole down inside the lowest part of the toilet that you can see. The hole faces the wall so its not easy to see but tsick you finger in there. If you are from Illinois you can wear a glove but feel around and make sure lime or calcium has not built up block the hole that creates the siphon effect.