Help!

[val1]

I woke up this morning to see my ammonia through the roof. The tank is all cloudy looks like a cycle (not sure why). The tank has been up and running for 18 months and have no new additions in quite some time(at least a month)
The levels are as follows:
ph 8.2
NO2 0
NO3 15-20
salinity 1.026
Water changes every two weeks 30 gallons
The tank is 240 gallon and a 60 gallon sump/ refuge
So that's it please help me find out what is going on. For now all the fish look good and are still eating very well.

 

[Kengaroo131]

check for all your live stock something might have gotten sucked through a power head do a large water change once every 2 days untill you notice a differance.

 

[val1]

So after 4x60 gallon water changes things are back to normal with no fish lose at all. This is the second time this has happened and I am not sure why, it seems to happened every few months or so. Does anyone have any idea why this happeneds?

 

[Monstrigity]

Usually this happens when something big dies and starts decomposing. Either that are someone dumped a truck load of food into your tank. The only other likely possibility is that something is comprimising your biological filter, Copper, formalin ect...