First have a look at this concept on the wiki.
Now you need another information: You can enable/disable any effect through events.
You have two possible ways:
1) Make an options menu where the player may choose different quality settings for the game. When the game runs too slow, the player lowers the quality and the game runs better afterwards. Behind the scenes you apply a low and a high quality effect just as described in the wiki, but instead of automatically switching according to the shader version, you enable and disable the effects according to the quality setting.
2) Constantly check the frame rate. As soon as it falls below a certain threshold, you disable all the high quality effects and enable the low quality ones. As soon as the frame rate is high enough again, you reverse it, disabling the low quality effects and enabling the high quality ones.
But always remember: pixel shader effects are realtime computing algorithms. They compute every pixel of the object or layer they are applied to on every frame. Don't use pixel shader massively, use them rarely and just to stress a certain aspect of your game. That is much more impressive and also the way even the newest hi-end games do it.