One way to "decrease" spells like nova or plague would be to have them hit only half as many targets. I.e. each creature, quanta pool, etc. that would normally get affected would have a 50% probability of being missed.
So, for example, when nova is cast, each quanta pool would have a 50% chance to gain +1 quantum.
When plague is cast, each enemy creature would have a 50% chance to receive 1 poison.