07-12-2011, 08:27 PM
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First, welcome to Tropical Fish Keeping forum.
Now your question. The fish must be slowly re-adjusted to "better" water. If not, the shock can kill them outright.
As tanks run with fish, the organics accumulate in the substrate. Bacteria break these down, producing CO2 (carbon dioxide) in the process, and this tends to acidify the water by adding carbonic acid. This is perfectly normal. The carbonate hardness (KH) of the water has a buffering effect on pH, keeping it stable. But at some point that capability is exhausted, depending upon the hardness of the water, and the pH then falls.
This is a slow process and fish adjust to it. At the same time, nitrates are likely increasing. Ammonia is also being produced from the breakdown of the organics, but if the p|H during this becomes acidic, ammonia converts to ammonium which is harmless to the fish. Again, this is all relatively slow, and fish adjust.
If you suddenly do a massive water change with "fresh" water, it will likely be very different chemically from the tank. If the pH of the added water is basic (above 7.0), the ammonium immediately turns into ammonia and will likely kill the fish. There are several factors in all this, but the point is that a massive water change to clean things up can be deadly.
It is best to replace a bit of the water slowly, vacuuming the substrate a bit each time, over a period of several days.
There will be other issues with the water too, and they also are best if things are changed slowly.
Moving the tank, keep the filter wet with tank water and when you set it up again rinse the filter media with tank water, not tap water. Use a good conditioner, in this case Prime would be best as it handles ammonia, nitrite and nitrate.
If I've missed something, someone else will probably catch it. Feel free to ask questions.