For a tank that size, 3 weeks is really really short compared to the time scale that we have to think about in this hobby. meaning, because how big it is, any one "wrong" thing that's happening in your tank may take a couple of months to rear its ugly head, this is true especially for new tanks. in this regard, saying that "everything's been fine and thriving for 3 weeks" isn't really giving you any real useful information on if your inhabitants are on a positive trend or slowly decline.
Most organisms have internal mechanism to maintain homeostasis as much as it can, so outword appearance will not change much, until it can't and you have sudden crashes. That's what
@zoa what and others are saying.
Thats why testing is important, b/c otherwise you really have no handle on what's really happening in your tank and the "eye test" is notoriously inaccurate.
To further the "complication", if your rocks and substrate are new, a tank that young wouldn't even give you a complete picture b/c a portion of the nutrients and other things you are testing may be absorbed in the rock and substrates so they dno't show up in water tests, until the rocks are saturated and suddenly you get a second round of "things go out of control" 8 - 10 months later. This is often why we recommend to take things slow in newer tanks.
The above are all generalizations obviously so there's going to be tanks that maintain success by doing something different. but again, generally speaking, doing the above will give you a greater chance of long term success.
