OK, if the UV was off it could have been bacterial bloom of heterotrophs. The problem is the bloom would of lasted 24-72 hours and would of caused cloudy water. It would of had the ability to remove large amounts of dissolved O2. This could cause a mini crash but you would of definitely seen cloudy water.
The new bulb could of done 2 things. One, began to UV burn the soft corals leaving them to leach toxins in the water as they are being damaged.
Two, it could of caused an overproduction of zooanthelle which would cause O2 poisoning. This only happens when guys upgrade to 1,000w bulbs from 250w bulbs and is rare.
The clams are small and probably didn't spawn. However if clams spawn in an aquarium they turn the water a milky white and release tons of sperm and eggs at once and again use up alot of O2 when it happens.
If water changes are fixing things and water params are ok and the light is working Ok then you may of just had an isolated incident of misfortune. If the problem does not reoccur then trouble shooting will be just guessing.
Think about everything that took place 2 days before the incident upto the incident and see if anything was different than everyday routines.(anything).