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Oh, that’s a very accurate test kit, so getting a 0ppm reading is not good for your system.The Hanna ulr
The natural reef has 0ppm phosphate readings, but there is a constant food supply that the corals catch 24/7. When people simulate a ULNS (ultra low nutrient system) they keep the residual P:N levels low, but they are feeding and exporting very heavily.
If you only have a little fish and are lightly feeding AND having 0 residual readings, it is a recipe for bleaching corals.
Luckily for you, the solution is clear and simple:
Add more fish
Reduce filtration
Feed more
Initially the phosphates you add will get absorbed pretty rapidly in the calcium carbonate (your rocks) so you may need to dose bottled phosphates as it will be much easier. If not, it will take a lot of food to get that initial reading, and you may get bacterial blooms from the sudden excess feeding, which adds carbon as well as phosphates which fuel the blooms.
So, to recap: get some phosphates into the tank. At least 0.03ppm. Initially, dose phosphates in a bottle, then maintain with more feedings/reduced filtration.

