07-07-2007, 07:38 AM
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Doing such large water changes are fine IMHO. The beneficial bacteria barely lives in the water, and really lives in the filter, as well as the substrate, ornaments and on the glass etc. What I would say when doing water changes like that is to do it reasonably quickly because the bacteria exposed to air will die if it dries out so as long as you do it quick ehough for the tank not to dry you'll be fine. If you're worried about that then do 30%, fill it up, and do 30% again straight after. This is what I do and I have never had a problem with params.
Doing large water changes doesn't mean that it is overstocked or overfed in all cases althoguh sometimes it can. Some fish are messy eaters, or produce a lot of waste. Also some foods make more of amess than others. My plecs love veg but some of the veg makes the water dirty very quickly the way they annihalate it when I put it in.
However if your tank water isn't getting very dirty, 30% will be all that is needed. If your params are high, rather than just getting a reading then I advise larger water changes, if not 30% will probably be sufficient. You'll be able to know for yourself how much is needed if you check again shortly after doing a water change. If you havent brought them down much, just change some more. Then continue this every day or every second day.