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—3天，或燥晒1天（如果急用可按每100公斤水加入0.63克大苏打，即硫代酸钠即可立刻使用）。其次，氯胺虽然文章不常提及，但这也是养好鱼的关键问题之一。在某些地区的水质中有较高浓度的可溶有机物，如果此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，可以利用生物过滤去除（在这里要提醒刚买新缸的朋友，为什么许多朋友在换新缸时很容易死许多鱼，就是因为，新缸中的生物分解过滤没有形成，在这种情况下氨的浓度会越来越高，以至会杀死你的爱鱼。最好的方法是使用沸石片去掉水中的亚硝酸盐）。

接着，我将谈谈PH值的问题。如果想增加PH值使用碳酸氢钠（小苏打），如果想降低PH值使用磷酸二hydrogen钠。注意同时使用PH试纸，每天调节PH值的度不要超过0.5，但是你要知道，0.5对于鱼来说相当于water里的acid或alkali增加了50倍……最后提一下：不断为water充氧可以起到稳定PH值作用。

然后，我将讨论适当的fish数量。你也许会问，这和water quality有什么关系？回答是当然有的，而且有相当大的关系呢！fish类越多排出的废物越多，污染、疾病、死亡随之而来……基本原则：平均10升water养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，我将简述nitrogen循环的问题。当fish放入aquarium，它们就会排出ammonia and other waste to water, water ammonia concentration will rise, water some beneficial bacteria will decompose ammonia,(注意: these beneficial bacteria group formation need a week or longer time), as long as these beneficial bacteria groups form, ammonia concentration will decrease until zero。但事情并不这样简单，因为 these beneficial bacteria only convert ammonia into nitrite but is toxic to fish! So everyone must understand that when ammonia concentration decreases while nitrite concentration increases in the meantime.BUT another kind of beneficial bacteria will form groups and convert nitrite into relatively non-toxic nitrates but this process also needs a week or longer time.The final product nitrates' concentration is controlled by water change, which is the most important reason for changing water.

以上所说：the entire process from ammonia to nitrite and then to nitrates just called nitrogen cycle.To make a new aquarium achieve good nitrogen cycle state needs 2-3 weeks' time.

If your biological filter has not formed yet and your fish appear problems you urgently need to treat the water:

For Ammonia:

1., replace part of the water (lowering concentrations)

2., add activated alumina (absorb Ammonia) in the filter

3., add helpful bacteria.

For Nitrites:

1., replace with clean water.

2., use one teaspoon of salt per 4.5 liters of aquarium volume (although it can't lower concentrations but in emergencies can neutralize toxins; salt won't harm fish).