The amount of water change needed and how often depends on how fast fish pollution and plant detritus is building up in your aquarium. Every tank is different and the more stocked your tank is, the more amount of water needs to be exchanged. So, you need to ask yourself these questions:
How stocked is your tank? Are you close to, at capacity or overstocked? Is there a lot of plant detritus floating around? (dead leaves, etc.)
Also, if you are dosing any fertilizer (if you're starting CO2 you'll need to dose ferts of some kind) it is necessary that you do water changes more often to clear out the uneaten ferts that your plants don't use. Personally, if your tank is well stocked I suggest 30% weekly, or 50% biweekly. Some recommend 50% weekly changes when dosing ferts, but it varies in opinions. Smaller water changes more frequently is usually better for the overall health of your tank. You should also be keeping an eye on your nitrites and ammonia, both ideally kept at 0. Yes, it is ok to have some nitrates (some say 80 and under, I say keep it under 40 at least).
“Alive without breath,
As cold as death;
Never thirsty, ever drinking,
All in mail never clinking.”
― J.R.R. Tolkien
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Last edited by Ashnic05; 02-19-2013 at 05:48 AM.