Ammonia, Nitrite(NO2) and pH are the three most essential tests to following along during cycling, however Nitrate(NO3) can be more helpful later to help serve as a check on the effectiveness of your regular gravel-clean-water-changes in maintaining a good water chemistry.
In a fish-in cycle of a 180L with a small number of zebra danios (4, 6?) if you test twice a day (every 12 hours or so) and are able to maintain the poisons, ammonia and nitrite(NO2), below 0.25ppm at the max then its likely the danios will not sustain too much permanent gill and brain damage and will not have their lives shortened too much. Your goal would be to use the morning and evening ammonia and nitrite results to decide how to adjust your percentage and frequency of water changes such that you never go over the 0.25ppm before being able to be home and perform another water change. It can sometimes be tiring work keeping up with this for the necessary month or two, if things go slowly. But if the number of zebras is small relative to the large 180L water body, the changes might not have to be too frequent. Once you acheive solid double-zeros for the toxins, though, the stock introduction rate will still have to be slow at first as the bacterial colonies will still be fairly small and fragile.
~~waterdrop~~
