12-11-2008, 06:45 PM
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A few comments: as the article said, ammonia and nitrite are deadly to fish, so both of these should stay at zero at all times. Nitrate is much less harmful but it doesn't hurt to keep it as low as possible. If you can get it down to 5 ppm, great, but 10-20 ppm is a more reasonable level. Once it gets above about 40 ppm, you've got a problem and need to start doing some water changes.
Different fish prefer different hardness levels. Some, like Amazonian fish (especially things like discus) need incredibly soft water. Others, like livebearers and especially the African rift lake cichlids, prefer extremely hard water. This goes for pH as well - different fish prefer a broad range of pH values. However, hardness and pH are usually tied to what's coming out of your tap at home, and it's almost always better to keep your pH and hardness where they are rather than messing with them as pH swings can kill fish.