I agree with Andy. There is always some ammonia in the tank, it is just too low to register on our testers. With Seneye being able to measure tiny amounts, it can see this trace amount.
The 'ammonia' will be NH3 + NH4 together, like with test kits. NH3 in water is in an equilibrium, which I can't type properly as I am missing symbols on my keyboard. (I'll have to use the symbol for reversible equation instead of equilibrium)
NH
3 + H
+ <-> NH
4+Ammonia and hydrogen ions are constantly joining together and separating, and the amount that are in the NH
4+ form at any given instant is temperature and pH dependent. The more acidic the water, the more H
+ there is - this is the definition of an acid. With an equilibrium, the more there is on the left side of the equation, the more it is pushed over the right side. So increase H
+ on the left (ie making it acidic) and the more it is pushed over to the right, into the less toxic NH
4+In basic water - pH over 7 - the amount of H
+ in the left is lower, so the equilibrium is displaced towards the left, so there is more NH
3 and less NH
4- at higher pH.
I can always complicate things further by saying where the H
+ comes from but I'll leave that for now
pH varies during the day particularly in a planted tank. Plants take up oxygen and make carbon dioxide 24/7 in respiration just like we do. But they also take up CO
2 during daylight during photosynthesis and use this CO
2 to make carbohydrates. The waste product of photosynthesis is oxygen.
The net effect is CO
2 made all the time but CO
2 taken up during daylight. There is more CO
2 in the water at night than there is during the day. When CO
2 dissolves in water it forms carbonic acid. So tank water is more acidic in the dark that it is in the light - the pH is lower at night than during the day. At 7 am the pH should be lowest because all the fish and plants have been 'breathing out' carbon dioxide during the night so there is more of it in the water at the end of the night than at any other time. As soon as it starts to get light the plants start to photosynthesise and remove some of the CO
2 so the pH starts to rise.