Step 1....use 10ml of water from your tank
Step 2...press start
Step 3...add in the Phosp reagent
Step 4...press start again and wait for timer to stop
(If at step 4, there is no timer appears, then you have to re-start step 1 using new tank water instead of the current tube)
But you can also using a fresh tube and perform the step 1 first....so you do not waste a packet of reagent,
Hope this can help you.