I suggest looking at bristlenose plecos, the albino ones are cute and only get to 4 inches. As for the nitrogen cycle its not about the water being the same its about balancing the chemicals to make sure your fish lead a happy and healthy life.
In basics fish produce waste which gets turned into ammonia very quickly, this can poison your fish. When the ammonia is broken down it gets turned into nitrites which is also very poisonous to fish but then the good news, is that in a cycled tank it will turn into nitrates fairly quickly which is harmless in small quantities, so regular water changes will keep your fish and you happy!
I suggest buying an ammonia, nitrite and nitrate
test kit and some pure ammonia (or even fish food can do it I think). Add the ammonia/fish food daily and test the water daily. You should see ammonia readings, then nitrite readings. When you have readings of 0 ammonia 0 nitrites and about 5 nitrates you are ready to add and enjoy the fish!!.
hope this makes sense!