As mentioned, turn it off, remove the screen or unit and scrap it over the trash or toilet.
The more you removed by hand the more Excess nutrients you remove from the tank.
Eventually it will start to come into balance.
Do you have any test kits yet?
Most people recommend Salifert, Red Sea, etc.
API is fine for fish only and softies. But not accurate enough at low levels for NitrAtes. Phosphates I use the Hanna checker.
You can probably get your LFS to do everything but phosphates or maybe even those.
I'm much prefer that because I hate testing.
Phosphates are usually not included locally.