11-10-2010, 12:49 PM
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The quick answer to both questions is, no.
On the issue of tap water/distilled water, it can somewhat depend upon the parameters (hardness and pH) of the tap water compared to the tank water. If for example your tap water is very hard and basic [=pH above 7, hardness high] but you have fish that require soft, acidic [pH below 7] water, the tap water has to somehow be softened and acidified, and one way is by diluting it with distilled water, rainwater (if safe in your area) or more usually RO (reverse osmosis) water. Pure distilled water has nothing in it, and no water exists in this state in nature so no fish are accustomed to it. But mixing part distilled in with part tap will dilute the hardness of the water, although this depends upon what is required for the specific fish. In a tank of livebearers or rift lake cichlids, which are fish that require basic harder water, it would serve no purpose. I should also point out that with the lack of any mineral in distilled water, it has no natural carbonate to buffer pH, and this can cause considerable trouble in an aquarium with fish, as the natural biological processes work to acidify the water and without any means of buffering the pH can crash badly and some fish can be killed outright, or at the least severely stressed internally.
As for the second question, it depends upon what is in the tap water. Almost all municipalities use chlorine, and chlorine will dissipate out of water left standing for 24 hours; you can also remove it quicker by brisk agitation of the water. However, there is more to all this that just chlorine.
Many municipalities also use chloramine now, and this is tied to ammonia and cannot be removed by the afore-mentioned methods--which is why they use it, it retains its effect (unlike chlorine) as the water travels through the system. The only way to safely deal with chloramine is to use a water conditioner that detoxifies chloramine (and chlorine).
There may also be other substances in the water, various heavy metals for instance; although these are normally in "trace amounts" safe for humans, these amounts are in many cases quite unsafe for fish. Copper is one, and if you have copper water pipes in your house this is a possible source of copper in the tap water. A good water conditioner that also detoxifies heavy metals (most but not all will do this) is also recommended.
Last, there is the effect of water changes on fish. We all advocate the positive benefits of water changes, and this cannot be denied. But, significant changes to the water can be detrimental to the fish, and again most good water conditioners will contain elements that protect the fish's skin and related benefits.
Given the relatively low cost of several "good" water conditioners, it is simply not worth the risk to avoid using one.
Hope this helps.
Last edited by Byron; 11-10-2010 at 12:53 PM..