Why A Big Water Change.

It is simply because all the ammonia you have added during cycling is turned into nitrite then nitrate. The big water change is to remove all that nitrate and get the level down to as close as possible to the tap water nitrate level.

It used to be thought that nitrate in the hundreds would not harm fish but it has been shown in the last couple of years that nitrate above 20 does have an effect. So the lower you can get nitrate before the first fish go in the the tank, the better.

Of course some of us have tap water nitrate well above 20 - UK legislation allows up to 50 ppm in drinking water - so water changes won't get it lower than whatever comes out of the tap.

In general terms, rather than ammonia cycling - water changes remove the nitrate made from the ammonia excreted by fish, though if there are lots of live plants there won't be that much since plants use the ammonia. But fish excrete other things as well, things we can't test for. Once upon a time I worked in a hospital chemical pathology lab, and we tested human urine for all sorts of things. Fish secrete similar, if not the same, things which build up in the water. Fish communicate by chemical signals - pheromones are for communication within a given species; allomones are for communication with other species. When levels of these build up in the water they cause stress for the fish, and we all know that stressed fish get sick more easily.

In the wild, rivers and lakes have constant water changes as the water flows down rivers, and into and out of lakes. The chemicals excreted by the fish are constantly washed away. Water changes in fish tanks are the next best thing.

Your tap nitrate is very good news  You'll be able to keep the tank nitrate well below 20 ppm when there are fish in the tank.
And with reference to your other thread, since you are not quite cycled yet you'll be adding more ammonia which will make even more nitrate. 1 ppm ammonia is turned into 2.7 ppm nitrite and on to 3.6 ppm nitrate.

1 ppm ammonia is turned into 2.7 ppm nitrite and on to 3.6 ppm nitrate.

Sue, can you please provide me with a link to that equation. I know it's correct, but I've looked for it a few times lately to support what I've said in various places and simply cannot find it.

Fish also produce pheromones (allomones?) that inhibit the growth of others of their own species, and often also closely related species. These are also removed by water changes.

@Hampalong - it's just simple chemistry and arithmetic.

Unfortunately Sue, the words "chemistry" and "simple" do not go together.

But that is a remarkably simple explanation. Maybe my old chemistry teachers just couldn't explain it very well. Education back then was more about learning than understanding.

When I did physics A Level, I couldn't get my head around e=mc2 it's just not immaginable. Break something and energy comes out. That like bitinf a pieve of toast and there being an explosion.  If I can't see it I don't understand it (because I'm weird like that!). Chemistry therefore is not my topic of choice!

Totally get what Humpalong means by learning not understanding. I just learnt the formula and did my test... never understood it. Neither did I get a good Mark, not like the topics I did properly understand 

I'm a geographer. Rivers, mountains etc... things you can see! 

Engineers, we have only just grasped the need to wash hands going into the toilets as well as on the way out Happly use you scientist's ideas though