Since writing about test kits, I have been paying a bit more attention to where people go wrong and I have slightly revised my recommendations on the importance of various kits for beginners.
Ammonia and nitrite
Ammonia and nitrite are still top of the list because ammonia is harmful to fish and nitrite causes haemoglobin to convert to methaemoglobin, which effectively suffocates fish as they are no longer able to absorb oxygen.
GH and KH are general hardness and carbonate hardness respectively. Both types of hardness are measured in (German) degrees, where 1° is 17.848 ppm. 1 ppm of hardness is equivalent to 1 mg/l (and also one American degree, but this should not be used as ppm or mg/l are simple enough and considerably more common). Water hardness comes second because fast changes in water hardness are quite likely to harm or kill fish and this is something that many people are not aware of. One can avoid these changes by always drip acclimatising, especially if one is not able to measure them.
pH and nitrate
While nitrate in itself is unlikely to be lethal to fish in the concentrations that are most often seen in aquaria (up to 500 ppm), there is scientific research to back up the popular consensus that it is best to keep nitrate as close to 0 ppm as possible because it will harm the fish (for example, it will result in stunted growth). Nitrates will rarely rise above 50 ppm in a well maintained aquarium even if they are not monitored as they will normally be used up by plants and removed through water changes. pH is important, but not as much as as water hardness because a difference in water hardness will indicate that there is a difference in pH, but the KH also indicates how stable the pH is.
Most “master” test kits do not include the water hardness kits, so it is best to double check what you are getting if you buy a set. I am still working through the kits that I used for the 60 litre aquarium, even though I have been using them for all of my aquaria, so they really are not all that expensive at around £30 for the set and well worth the money to save beginners a lot of headaches.