well you can do what the book says if you want but when you change that much water on them they cant tolerate it as well a lil at a time is ok like once a month but me I mostly just add water and clean my gravel to change water and my fish do great and even better since I started using salt...but I wouldnt do a 25% change and a 50% in less then to weeks that means your changing almost all the water in the tank it leaves none of there normal habitat you should look up on the net about changing your water I'v always found more on the net then trying to find it in a book I hope I'm helping and not making things worse oh and like I said if your tank has been used for animals before fish that also might be the problem

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