Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Sodium or potassium nitrate to lift nitrate off the floor. Ultra low nutrient tanks starve corals and grow dinoflagellates.
Raises 284 L by 4.50 ppm nitrate using Sodium nitrate NaNO₃. Dissolve it in tank water first. Dosing dry salt straight in creates a local spike that nothing benefits from.
grams = (5.00 − 0.50 ppm) × 283.9 L ÷ 729.5 = 1.751 g
Sodium nitrate NaNO₃ has a molar mass of 84.994 g/mol and the nitrate ion is 62.004 of it, a mass fraction of 0.7295. One gram in one litre gives 729.5 ppm nitrate, so the dose is ppm × litres ÷ 729.5.
Potassium nitrate KNO₃ weighs 101.102 g/mol for the same nitrate ion, a fraction of 0.6133, giving 613.3 ppm per gram per litre. It also adds 3.87 ppm potassium for every 10 ppm of nitrate, which is usually harmless and occasionally useful.
Doses are small enough that a jeweller scale reading to 0.01 g is worth owning. Below that, make a stock solution and dose by volume instead of trying to weigh 0.2 g on a kitchen scale.
Tanks running nitrate at zero are the usual home for dinoflagellates. Bringing nitrate up to a few ppm is one of the standard first moves against them, alongside stopping the aggressive export that got the tank there.
Corals need nitrogen. A tank with pale, slow, receding tissue and undetectable nitrate is usually starving rather than diseased. Zero is not a target, it is a warning.
Nitrate and phosphate work as a pair. Raising one while the other stays at zero rarely helps, and the ratio matters more than either number alone.
Sources: Dieck, Reef Chemistry Calculator