[bertoni]

The nitrate method might work. One of the phosphate methods might be fine, although it'd likely require recalibration if it's low-range. The hardness won't be useful. It at best will give a sump of the calcium and magnesium, but the level in tank water probably is far out of range of the kit. That device is largely for freshwater testing. There are companies working on sensors for saltwater systems, but I haven't seen a lot of reviews yet.