Doing a pwc only when some test indication is off is not good aquarium management; and understand, I am not referring to the obvious ammonia and nitrite, but others like pH, nitrates. By that time the problem is probably far advanced, or at least the early stages have set things in motion that would otherwise not have occurred had a regular maintenance schedule been adhered to as it always should, whatever that maintenance might include. Also, such drastic changes (depending upon what the "test" is for) can cause more harm to the fish, and possibly the plants, than doing nothing. And the ramifications of such issues are frequently long-term and may go unnoticed until weeks or months from now and then be too late to remedy.
There are a couple of threads on this forum, in Freshwater Aquariums I think, on this topic, with some very knowledgeable answers. I don't really want to wade into this discussion yet, but since your stated intention is more detrimental than not, I decided I need to say something.
Water changes replicate what all fish have in nature, constantly changing fresh water. No fish in nature (with very few drying pool exceptions) lives in a closed system comparable to an aquarium. The water is always moving past, be it as slow currents in creeks and streams or convection currents in ponds and lakes creating movement and fresh water. Plus the fact that there will never be the concentration of fish to water volume in nature as in an aquarium; even if you only had one neon tetra in a 100 gallon tank, the fish is still in a closed system that is nothing like nature where it came from, directly (wild-caught) or in ancestry (tank-raised commercially).
Second issue are the substances that build up in closed systems that again would never occur in nature; these include both toxic substances from the fish and bacteria, but also other things we might not otherwise think of. Iamntbatman wrote well on this aspect in one of the other threads, I'm not going to repeat all that, and I certainly agree with it. This was the thread: http://www.tropicalfishkeeping.com/f...o-water-38289/
If the aquarium is not very soft water (near-zero hardness) then it has minerals in it which do get depleted over time; these are needed by the fish and plants. The only way to replace them is via a water change. Consistency in water means better health for fish and plants.
And this gets us to the part about water changes involving how much and how often; and on this I am not going further at this time, since I have written at length in other threads.
A related thread with good info: http://www.tropicalfishkeeping.com/f...-answer-38618/