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sovrappensiero 02-21-2011 12:25 PM

water change: tap & tank water parameters different
Hi again everyone,

It's been awhile since I posted a question, but I'm back. My tank is a 45 gallon, moderately-heavy planted tank that houses about 15 corys and a school of cardinal tetras (14 or so). Everyone's happy and the tank's been running since last August (I added the fish a few at a time, most recently 4 panda corys a few days ago).

consistent parameters: pH=6.6, ammonia=0, nitrite=0, nitrates=5ppm (it seems this is stable as long as I keep some DIY carbon dioxide running for the plants).

My problem is this: I wanted to do a 30% water change today after feeding frozen bloodworms yesterday and frozen brine shrimp today. I checked my tap water parameters first, and the pH is 8.0, ammonia is 0.25.

What should I do about the water change? Buy distilled water this time around? If I just do a 10% change, would that be ok? I've got new panda corys, like I said, and I don't want to stress them out.

BTW, fluctuating pH seems to be a common problem with my municipality's tap water. That's why I check it before I do a water change. Never seen the ammonia above 0 though.

Appreciate any help you guys can offer! Thanks in advance!

Byron 02-21-2011 06:06 PM

There are a couple options, and to suggest the best can you first tell us the hardness of the tap water? The water suply folks can tell you this, no need to get a hardness kit. The GH and KH, or whatever they can tell you, will help point us in the best direction.

While you're there, any idea as to why the pH fluctuates so much. And can you give us the spread for that?


sovrappensiero 02-21-2011 09:53 PM

Ah sorry - I guess it would have helped if I had provided the hardness info.

This morning I measured it using a test kit: GH = 2, KH = 2-3 (hard to tell; it was orange-ish at 2, but fully orange at 3 drops). My tank is GH=4, KH=2-3.

The usual spread is 7.0-8.0. I haven't tested it at higher or lower than that (doesn't mean it hasn't been higher and lower, of course). It's probably because the water is so soft...but my tank water's pH doesn't fluctuate this much (6.6-6.8, max 7.0). My fish don't seem to be bothered by it, but maybe I'm wrong....I tested once a day for a few months but recently stopped and started testing only once a week.

I really appreciate any advice on this. I thought I'd ask before it really gets down-to-the-wire on the water change...

Byron 02-22-2011 10:23 AM

The hardness is low so this is not going to be problematical.

First, something I should have mentioned yesterday, when testing pH of tap water let it sit out 12+ hours. The CO2 in the tap water can cause faulty readings, but after it dissipates out, the true pH will show. Depends upon how much CO2. In my tap water, this only makes a difference of .1 so I don't bother with the wait; after your fist test waiting you'll know.

With hardness that low the pH in the tank will naturally lower and tend to remain low. Water changes with equally soft water (as you have) will not affect the hardness, and the pH will rise a bit but then return to normal. If this is only by a few decimal points, it is normal and not a concern.

In my tanks for example, with very soft water (tap and tanks), the tank pH runs around 6 to 6.2, and my tap water is 7-7.2. A 50% water change which I do weekly only raises the tank pH by .2 or maybe .3 at most. Overnight it returns to normal.

I would do your normal water change, checking the tank pH before and then about an hour after the change. If the difference in pH is minimal, not an issue. If the pH in the tank raises above 7, in your case, I would thereafter do smaller volumes and more frequent water changes to compensate. Shifting the pH above 7 has another impact related to ammonia, which you also mention.

In acidic water, ammonia automatically changes into harmless ammonium. But if the pH rises above 7, it automatically changes back into toxic ammonia. Keeping the pH below 7 will result in no ammonia/ammonium issues.

You also have plants, and they help in all of this. They grab ammonium (or ammonia if basic water) fast, faster than the bacteria can. They will also help you on the water change, because the more plants there are--provided the fish load is not excessive--the less water changes are necessary because plants do filtration jobs that no filter can do, only the water change, so live plants are like mini-water changes up to a point. I wouldn't omit the weekly water change completely, but with lots of plants you can change less than otherwise.

Hope this helps.


1077 02-22-2011 11:03 AM

Maybe DIY (yeast ?) CO2 bottle is getting low, This might allow pH to rise.

sovrappensiero 02-22-2011 12:06 PM


Thanks a million for your detailed advice. Reading of your firsthand experience makes me feel less concerned about impending disaster; what you say makes perfect sense. I'll try a tap water reading after letting the water sit for 12 hours. I think you suggested this to me last year when I was having some issues and I did it, but the data are in a computer that is currently out of commission (one of my darling loved ones spilled liquid in the keyboard...and it wasn't one of the fishies :lol:). Hmmm...maybe I'll look back through my old posts for reference.

RE: 1077, yes I do watch for that, but my concern at the moment is whether it is safe to do a water change while my tap water pH is 8.0. The pH in my tank doesn't seem to fluctuate much. Of course, now that you mention it, that will be an important factor to help keep my pH under 7.0 whenever my tap water ammonia level read 0.25.

This is the best forum. So many helpful experts! Where would the forum be without you all? :-) Thanks very much to you both.

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