I am going to put a twist on this that is not going to change our recommendation for Rox (Hi Rox and Welcome to TFF) but is the kind of tidbit I know our members are watching for:
There is not -necessarily- a problem with refilling 15% or less of the tank water with UN-conditioned cold tap water! Just like the cold will be diluted so much that the overall tank temperature will not drop more than 2 degrees (F/C, whatever), the chlorines and chloramines will also be diluted so much that only the most delicate fish would have much chance of being hurt by them. If you were to look, you probably find some experienced members on TFF who simply use straight tap water for small changes. BUT, note that they also usually have a mature tank!
There are two main reasons we highly recommend using a good conditioner here in the newcomer/beginner section: First, many, if not most of our beginners are in some stage of cycling their biofiltration. If their tank is only 6 months old or younger then the two bacterial colonies are still maturing (there is significant maturation out through 6 months and small amounts of further settling in for the first full year.) While the colonies are still maturing (and maturing means more than cycling) it is still important to protect the investment in time and effort you've made during cycling from possible damage via chlorine or chloramines. While even a normal dose of unconditioned water may cause some damage and setback to the bacteria, the real concern are those days when the folks at the water plant decide to "shock" their pipes (just like the pool managers sometimes "shock" a pool after a kid has an accident.) They sometimes inject a larger dose of one of the chlorine containing products and it can make it all the way down to your tap!
The second reason most of us just use conditioner as a habit is that it is really just very, very cheap insurance; so cheap that there's hardly a downside. If you use a high concentration conditioner, such as the Seachem Prime that we often recommend, you've really covered youself against the odd "shock day." This is why most of us would never bother to change the habit, even after our tanks are mature.
There is another detail to the conditioner thing. While we are still in the first 2 or 3 months (ie. still cycling) or even the first 6 months, it is really good practice to overdose your conditioner to the tune of 1.5x to 2x dosing. Many old-timers will actually tell you to overdose, saying any overdose can't hurt. I disagree with that because of some discussion with Dr. Hovanec, which basically boiled down to his observation or hunch that dosing conditioner higher than 2x caused a slow-down in Nitrospira reproduction. I don't believe it was a heavily tested thing but to me it seemed reasonable to take it as prudent upper limit barring further knowledge.
(Again, apologies to Rox here for all this detail. The real skill your question plays to is about the technique and idea behind the "gravel-clean-water-change" concept and others have made a start on that. It is a very important skill to understand and practice.)
~~waterdrop~~
