Probably A Stupid Question...

[xostrawberryox]

What could cause a huge increase in nitrate??

 

[dwarfgourami]

A sudden, maybe temporary increase in your tap water (these things happen), decaying plants, a dead fish, overstocking the tank, overfeeding.

 

[moeldner]

A reduction in water change frequency? Changes to your levels of tank maintenance?

 

[Silverlady]

Overfeeding?

 

[woogrl]

If alot of water has evaporated and you only "topped-off" since your last test will make it go up.

 

[xoedusk]

How huge of an increase and over what time?

 

[Don155]

Could be something died and got hidden somewhere where its decomposing. Check everywhere, including inside your filters if you can't find one of your inhabitants. Heck check inside your filters anyway, you never know what can suddenly get sucked in there by chance.