07-13-2011, 11:18 PM
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What are your water parameters? Ammonia, Nitrite, Nitrate and pH? What type of filter do you have? By dirty I assume you mean the filter media was full of gunk? If so, then just give it a quick rinse in aquarium water.
The filter media is where a large number of Beneficial Bacteria live and unless the filter media is literally falling apart, you do not change it.
Changing the filter media can bring on a mini-cycle leading to a buildup of Ammonia. The first step is to get your water tested, I recommend using the API test master kit. Once we know the number we can better assist.
For now do your fish look like their breathing rapidly, red gills and or are they hanging out near the surface?
If so these could be signs of ammonia poisoning and or low oxygen content. Consider at least a 50% water change. From then on let your water test dictate how much water to change.