04-04-2010, 07:21 PM
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Planted tanks and non-live plant tanks need different maintenance, so the first question is, do you have a planted tank or not?
If it is not planted, which I'm assuming from your post (pardon me if a wrong assumption), then you must do a partial water change every week to ensure stability of the water parameters and ensuring that nitrates are low. A lot of sludge suggests you might have a high organic load in the tank, or perhaps just haven't been doing weekly water changes. The gravel gets "cleaned" when you do the water change. Depending upon how much water you change, you can do the entire substrate every time or maybe half of it with the other half next week and so forth.
The amount of water changed depends upon the fish load; the more fish and/or the larger the fish, the more water needs to be changed.
Given that you should be doing this every week, removing the fish is very stressful for them, much more than the water change. After a while they rather get used to the water change. I've had my hatchetfish come up and try to nibble the hairs on my arm under water. The other day I was moving a few plants and wood around, and when I sat down to see what I'd done as I was working on it, some of the fish immediately got in the spot and checked out what was going on, looking for food bits stirred up, etc. Chasing the fish around, like with a net, is far more stressful for them, and stress means health issues are more likely to occur.
If further questions occur to you from this, ask away. And if you do have plants, I'll comment on the difference.