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静置2到3天，或燥晒一天。如果急用，可以按每100公斤水加入0.63克大苏打，即硫代酸钠即可立刻使用。此外，氯胺也是养好鱼的关键问题之一。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。

此外，PH值也很重要。PH值7为中性，大于7为碱性，小于7为酸性。如果想增加PH，可以使用碳酸氢钠（小苏打），如果想降低PH，可以使用磷酸二hydrogen钠。但是，每天调节PH值的度不要超过0.5，因为对于鱼来说，这相当于water里的酸或碱增加了50倍……最后提一下：不断地为water充氧可以起到稳定PH值的作用。

还有一点要注意的是适当的fish数量。你可能会问，这和water quality有什么关系？回答是当然有的，而且关系很大！fish类越多排出的废物越多，对环境造成污染、疾病、死亡等问题。此外，还应该知道基本原则：平均10升water养的fish总长度不超过5.5厘米（注意：体长标准是从鼻到身体末端距离，不包括尾巴）。

最后，我要说的是关于nitrogen循环。我们知道只要put fish into the tank，它们就会排出ammonia and other waste to the water, which will increase ammonia concentration in the water, and some beneficial bacteria will decompose ammonia（注意：these beneficial bacteria need a week or more to form）。 But things are not that simple because these beneficial bacteria only convert ammonia into nitrite but also toxic to fish! So everyone should understand that as ammonia concentration decreases, nitrite concentration is rising. However, another type of beneficial bacteria will form colonies and convert nitrite into relatively non-toxic nitrate but this process takes another week or more. The final product nitrate's concentration is controlled by water changes.

以上所述整个过程由ammonia to nitrite then to nitrate called nitrogen cycle. To make a new fish tank achieve good nitrogen cycle status needs 2-3 weeks of time.

如果你的 biological filter hasn't formed yet while your fish are having problems you urgently need to handle water quality:

Replace part of the water (lowering concentrations)

Add diatomaceous earth (absorbent for ammonia)

Add helpful microbes

For Nitrites:

Change out some of the water

Use salt for emergency situations (though it won't lower levels but can neutralize toxins)

This information should be useful for all aquarists who want their tanks healthy and thriving with happy fish!