If you could be certain that nothing ever changes in your tank, then it is true you have no need for a test kit. But, exactly how would one know that nothing has changed and everything is going smoothly unless they periodically check by testing?
I think this type of thinking leads directly to "old tank syndrome". A scenario where the fish keeper gets lulled to sleep by how well everything in the tank is just zooming along. (Meanwhile, the poo is building up producing ammonia. The nitrates are building up lowering the pH. Both of these conditions can be tolerated in conjunction with one another, as the lower pH keeps the ammonia from becoming toxic.) Then one day out of the blue, the keeper decides to do a large water change, because it has been so long. Then, the pH spikes up, the ammonia becomes toxic and the fish all start dying! The keeper says, well, it must have been the water change that killed them. Next time I won't do that! And it just gets worse. While it is true that ultimately, it was
that water change that killed them, the real cause was the lack of proper maintenance.
If that same keeper did even monthly tests, they would have noticed the pH slowly dropping, the nitrates building up and the ammonia starting to rise. This would have caused alarm whistles for the keeper that something was a miss and a water change would remedy it. However, they would know that the pH was drastically different and the nitrates were way higher than the tap water. And so, while doing this large water change (to remove the ammonia) they would refill very slowly, which would elevate the pH back up gradually and slowly acclimate the fish to the new parameters from the tap. And since the tank hadn't gone very long (1 month tops) without a test, the ammonia couldn't be too high, which means that the level would be well under control with the large change.
Beware old (properly read "out of date") aquarium books which don't fully explain the connections between all the different elements. Fish keeping has taken some leaps and bounds in the past 30 years. Library books don't always keep up. (I recently helped my brother move and came across an old book my dad had bought about 30 years ago. It was a 100 page book, that devoted exactly ONE PARAGRAPH to the nitrogen cycle and ammonia/nitrite poisoning. ONE PARAGRAPH!
It basically said that once the tank is cycled, the ammonia levels should be at zero as should the nitrite. But, there was NO MENTION of
how to get it cycled, or what that even meant. So, beware of old library books on fish keeping.
