OK, so the thing you've got to get through your head is that there is nothing wrong with tap water. As long as you condition it (treat it with a product that removes chlorine/chloramine such as Seachem Prime, and by the way I recommend dosing the conditioner at 1.5x whatever they suggest) and roughly temperature match (by which I mean you can use your hand to match the new tap water to the current tank water they have and it can be rough but should be done with enough care that it's maybe about right on or maybe a tiny tad cooler) then you can change out practically all of it until they are almost flopping around on the gravel (not that you'd do that but to illustrate) and the fish will be happy as can be.
Too many people somehow come in to the hobby with the idea, or get the idea, that water changes are a big shock and should be minimized! That's not true at all if you understand how to do good water changing technique. Rather, water changing is your friend!
Your goal as a Fish-In cycler is to be on the lookout for the right pattern. You test twice a day, about 12 hours apart usually, generally. You don't listen to anyone telling you to change this or that percentage per day but instead tailor your percentage and frequency based on both what you are seeing from your test results and based on the convenience of when you can be home to change water.
You can change more water at less frequent intervals or less water at more frequent intervals but the whole idea is to have the ammonia(NH3) and nitrite(NO2) keep being put back close to zero ppm (or often showing zero ppm) but then to catch it again before either of them manages to spike above 0.25ppm before you can change water again. It's not a disaster if it manages to surprise you and get between 0.25 and 0.50 some day, it just makes you change your pattern again (the pattern must necessarily adjust because the spikes will get worse, then better, over the several weeks to come.) By being an observant caretaker and doing the best job possible keeping the range between zero and 0.25, you are giving the fish what they need to minimize damage to gills and nerves.
Soon the fish-in cycling weeks will be over, you will see constant zeros and you will have an appreciation of this core aspect of the hobby for the rest of your life, a valuable learning lesson.
~~waterdrop~~
