Bad smell, turbid water and white (fuzzy) spots!

This is REALLY BAD!! ...Your tank is in (or is about to start) a massive decomposing event... commonly know as a "melt down".

Because the process of decomposition uses oxygen... when something big dies in your tank, it decomposes rapidly sucking up oxygen more rapidly that it can be replenished... so oxygen in the water runs out (anoxia). The lack of oxygen causes other living organisms to suffocate and die. This adds to the decomposing organic matter, which in turn exacerbates anoxia. The end result is a snow-ball effect that ends up in the TOTAL mortality of everything in your tank (with the exemption of anaerobic bacteria).

A massive decomposition event happens when you add lots of UNCURED Live Rock in you tank. The uncured Live Rock contains lots of dead matter that will set off the melt-down... it is unavoidable... you just want it to happen in place that is NOT your tank.

Symptoms

In order of appearance...

1. Coral polyps and feather dusters close up (they get cranky).
2. Other fish and inverts start behaving weird.
3. Water starts smelling rotten.
4. Turbidity increases (because of bacteria in the water).
5. Inverts and fish start to die.
6. Turbidity increases even more and water stinks really bad
7. A white fuzzy cover starts growing on the rock

Treatment

1. Take ALL the newly added Live Rock out of you tank.
2. Cure Live Rock.
3. Add back to tank
   

To avoid this problem

Don't add lots of uncured Live Rock in your tank. If you have a large and established tank, you can add one or (maybe) two pieces... the die-off will still happens, but as long as oxygen can be replenished fast enough and you have a way to deal with the ammonia and nitrates spikes... you should be fine.

Otherwise, you can buy cured Live Rock or you can cure your Live Rock at home before placing it inside you display tank (learn more about curing Live Rock).

White spots (not fuzzy)

(?)Source: Vancop, 3reef.com

This is cause by coralline algae dying because of high light. Many types of coralline algae like low light and therefore it may die when introduced in a new tank with lots of light. The white spots are the calcareous remains of the algae.

Symptoms

Coralline algae turns white...  causing white spots or patches. Note that the white spots are not fuzzy... they cannot be simply brushed away.

Treatment

1. Move affected rock lower down in the tank where there is less light.

You can also choose to sacrifice the coralline algae in the affected rock. New (high-light adapted) algae will take its place.