0 Nitrogen

Looks like I might have a problem with low or no nitrogen in my planted tank
(CO2 injected); currently tested Ammonia, Nitrite and Nitrare as 0 (Aquarium
Pharm test kit), some of the plants that take N from the water column such
as Limnophilia Aquatica is suffering (lower leaves are browning off), even
the Anubias are starting to look yellow in the older leaves. The fish load
in my 280L is still low at the moment, as I'm basing carrying capacity based
on expected fish sizes (currently 5 SAEs, 13 Ottos, 18 Cardinals, 12 Rummy
noses and 6 Congos). Besides feeding them more (and also getting more
phosphates and possible getting algae), any suggestions, as N appears to be
the limiting factor here? Does anyone know of a source of Potassium Nitrate
in Sydney?

Cheers,
Poe