Max % water Change
 

Okee..

Like the subject says... what is the safest percentage of water volume that can be changed at any one time for a tank?

Currently I do a 30% water change every week on my 20gal nano. So about 6 gals.

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I am trying to get my Nitrates down because of a problem with previous water changes that were adding Nitrates to the tank.

What are my options?

TIA

I've done 60% changes before with no ill effects. I would have no problem doing 50% regularly.

I had a similar issue with my old 20g.

I did a 100% water change on it. Anything less than that yields you a point of diminishing returns. I wanted those nitrates gone (skimmer had broken and I ran it a little too long skimmerless).

Anyhow, it was fine after that.

Of course, I had no fish to worry about (just corals and an anemone), so that would be a complication if you had fish.

Maybe not so practical on larger tanks, but on a 20g it's definitely something that's do-able (just need a little coordination and planning).

I have found that things get stressed if I do more than 50%. I do 25% weekly on a 50gal. I have almost undectable nitrates and phosphates so I don't see a reson to do more. Make sure you have a good salt and good food. What is or was the source of the nitrates and how high were they? Have you eliminated the source?

Rick

I agree 50%. I only do that rarely.

The source was my change water. It seems for some reason that if it is mixed without being used for more than 2 days it starts to cycle.

I did a test a little while back when I realized my Nitrates went up instead of down when doing a water change. So, my Kent salt water mix that was made for more than 5 or 6 days prior to changing the water had a nitrate level of over 25!

Currently my tank sits at 25ppm.

I now mix and change the water the day I make it.

BTW I do have an RO/DI and the RO/DI water stays nitrate free for over 3 days (the longest I tested) .. It only starts to cycle once I added the salt??? weird.

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It doesn't help that you salt was adding to the problem but I would begin by getting to a proper resolution for this issue. How much live rock do you have in the tank, and how much flow? I do a %10 water change every 3 weeks on my 54gal. (65 gallon with sump) and my nitrates never exceed more than 10 or 15 ppm, which I consider accetable after that alotted time. Your feeding habits and the amount of live rock\sandbed will determine how well your system can effectively denitrify your water. I agree that large water changes can be stressful to a tank so I am minimalistic in this regard. Furthermore, this is only what I have noticed from my experience with smaller tanks, consider adding more live rock to house nitrifying bacteria and see if you notice a difference. A tank that can heal itself is much more healthy than one that requires a doctor.

25ppm, not great but could be worse. Try the 50% change and see how the tank goes.

The problem with water changes as a means of nitrate control (or "management" if you prefer) is "diminishing returns." If you did a 50% water change one week, then your nitrates will be ~12ppm after that water change, so about a 12ppm reduction. Assuming no nitrate buildup and no other means of nitrate reduction, if you did another 50% change the next week now you only get a 6ppm reduction. And the next week only a 3ppm reduction, and so on. But the cost of doing that 50% changeout remains the same. That's why I feel when battling a tank "issue" you want to go as big as you feel comfortable with. Multiple small changeouts can't solve a spiraling problem (although they can be good preventative maintenance assuming everything else is in balance).

But .... for a 20g tank, water changes are less expensive compared to larger setups. A 50% changeout is 10g, and that's 2 5g buckets. Not bad. Gets a little more complicated when you're talking about using several 30g rubbermaids for one water change.

Curious though: Have you tested PO4?

Ok.. My tank has more liverock than can be explained. Its PACKED with live rock. Probably more than most people who have a similar sized tank.

Feeding, is almost NIL.. I feed my sun coral every couple of days and that is it.

Sand bed is about 1 to 2 inches in many places.

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I am pretty confident the Nitrates are not a issue where I was overfeeding or not having enough live rock or not skimming (which I do skim on my 20gal ) ..

I have 3 fish, a small neon goby, one damsel and a small clown.

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Not sure what more I can say other than for the past year I have been using change water that generaly had been sitting in a bucket mixing for more than 4 days. Because I had read it was good to let it mix for a few days. Over that time period I have been battling an ever increasing Nitrate problem. Which I could never figure out. Until I decided to test my change water and all of a sudden I found out I have been adding 25 ppm (or more or less) of Nitrates into my tank every time I did a water change.

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Tank photo to show live rock.. Hard to see but it is live rock right to the very front except for about an 1 inch or so on the right and maybe 2 inches on the left. Any more live rock and I'd have no corals.

http://members.shaw.ca/cmcd/20gal-Apr-28-04.jpg

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Well I guess I won't be using Kent sea salt; I might be inclined to call them up and compain!