我先来介绍一下自来水中对鱼类有害的一些主要成分和去除方法。首先，自来水中的氯气需要通过静置2—3天或燥晒1天的方式去除。如果急用，可按每100千克水加入0.63克大苏打，即硫代酸钠即可立刻使用。此外，还有一种名为氯胺的成分，这也是养好鱼的关键问题之一。在有些地区的水质中，有较高浓度的可溶有机物，如果这时水厂直接加入氯气，就可以与这些有机物结合形成一些致癌物质，对产生危害。因此，水公司在水中同时加入氯气和氨气，这两种物质可以结合成一种新的物质叫氯胺，而氯胺不与水中的有机物结合，所以当除掉水中的氯气后，氨还是在水中。这需要一个良好的氮循环，可以利用生物过滤进行去除。在新缸换鱼时，要注意生物过滤没有形成，在这种情况下，亚硝酸盐（NITRITE）的浓度会越来越高，以至会杀死你的爱鱼。最好的方法是使用沸石片去掉亚硝酸盐。
其次，我们要关注PH值，它决定了是否适合养鱼。如果想增加PH值，可以使用碳酸氢钠（小苏打），如果想降低PH值，可以使用磷酸二hydrogen钠。每天调节PH值的度不要超过0.5，因为对于fish来说，每增加0.5个单位，就是将整个环境变成了50倍强烈的情境。
最后，不断地为water充氧可以起到稳定PH值的作用。此外，适当放入fish数量也很重要，一般来说平均10升water只能养一条总长度不超过5.5厘米的小金鱼。太多fish排出的废弃物会导致污染、疾病和死亡。
此外，还有一点关于nitrogen循环化学变化的问题。当put fish into tank，它们就会排出ammonia and other waste to water, ammonia concentration will increase, beneficial bacteria in the tank will break down ammonia, but this process needs a week or more time, during which period fish are most vulnerable.
However, things are not that simple because these beneficial bacteria only convert ammonia into nitrite but is still toxic to fish! So when ammonia concentration decreases while nitrite concentration increases, another type of beneficial bacteria forms a colony and converts nitrite into relatively harmless nitrate but this process also takes a week or more time.
The entire process from ammonia to nitrite and then to nitrate is called nitrogen cycle. To make a new tank achieve good nitrogen cycle state requires 2-3 weeks' time.
If your biological filter has not formed yet and your fish appears sick due to poor water quality you need urgent action:
Change some of the water (reduce the concentration)
Add activated carbon (absorb toxins)
Add beneficial bacteria (break down toxins)
For Nitrites:
Change all the water
Use aquarium salt at 4.5L of water per 1 teaspoon of salt (it won't reduce it but can neutralize toxins; it won't harm fish)
Remember: Good maintenance practices include regular partial changes of tank contents every two weeks with fresh dechlorinated tap water that has been conditioned for use in an aquarium by adding products like "conditioners" or "stabilizers".