Bacterial bloom and Da Vinci Clowns

[jd371]

I started a 20g nem tank at the end of December and it cycled a few weeks later I then I started adding some livestock. Everything was doing fine until yesterday. I fed the clowns live black worms and some brine shrimp before leaving for work and they ate them with gusto. I went to feed them later that day and the tank was cloudy and the clowns were dead. Must of suffocated from lack of oxygen caused by the sudden bloom. I'll be taking care of the bloom with a WC and I have a small UV that I use for my QT tank that I'll run also.
The tank was crystal clear when I left it in the morning and cloudy that afternoon. Which brings me to my question, what could cause a drastic bloom in a matter of hours like this?

 

[EmdeReef]

Sorry for the losses.

Do you have a nem in that tank?

Given bacterial bloom’s positive feedback loop it’s a bit of a chicken and egg situation figuring out whether the bloom started before or after the fish died.

Have you tested your parameters after you found the fish dead? Any ammonia?

 

[davocean]

I'm not sure how you can tell it was a bacterial bloom even for sure, there are a number of possibilities.
Would need to know more about tank as to why this happened, but at this time WC's and carbon.

 

[jd371]

The tank is a 20g breeder started with dry rock and HOB filter running just some carbon and polyfilter material. Started at the end of December and cycled around mid to late January. Added some snails first 3 weeks ago followed by the clowns. I'll be adding anemones in a few months when the tank is more stable and I plan to move a rock from the main tank that has some RBTA's on it to this one also. The clowns were in the tank for about two weeks. I didn't QT them because they would be the only fish in this tank, I didn't observe any signs of parasites or illness while in the tank and they ate everything fed to them.
I tested the parameters last Saturday and the results were:
Salinity - 1.025
Mg - 1455
Ca - 425
Alk- 6.4 ( going to bring it up with some WC's)
NO3 - 50
PO4 - haven't tested yet
Temp 77-80 ( tank is in my small workshop that has the home furnace in it so the ambient temp in the room fluctuates. I do have a controller to turn on a fan over the tank if the temp goes above 80 )
NH3 - 0 ( tested right after finding the fish dead)