What causes pH to keep dropping?

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philip02180

AC Members
Nov 10, 2005
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Stoneham, MA
For the last couple of weeks I notice my 15g tank water pH is close to 6.0.
During water change I didn't even bother to lower the pH hoping I can slowing raise it back to my preferred level (~ 6.8, local tap water here is ~7.4). But no, after a day or two it'll drop back. What can cause the pH to stay so acidic? I think the plants are dying because of that. Could it be decaying plant leaves causing that? Luckily the fish are OK (I think!). Here's my tank spec:

15g
power head
power filter
airpump/airstone
Flourite + gravel subtrate
1 driftwood
plants: dwarf sag., hairgrass, micro sword, moneywort, amazon, bronze lily
fish: head-tail-light, neon, pleco
 
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The diftwood naturally causes ph to lower, also decaying plants will also could cause it to drop
 
As the bacteria aids to eliominate ammnia and nitrite, carbonic acid is released thus acidifing the water. This is normal. However, carbonate harness or Kh buffers the water so ph will not decrease so quickly. That's why soft water is almost always acidic and hard water tend to the alkaline side.

Fisrt of all, I'd recommend to measuree your kH (if you don't have a test kit, get one). Later, repost the resuls.

In the mean time, you could test your tap water pH right out of the tap and after letting it sit overnight in a plastic clean bucket. Sometimes pH varies over the fisrt 24 hours after it comes out of the tap. If it reamains stable, your problem may come form other side.

You could also try to remove the driftwood, as this sometimes causes water to acidify.

Ph drops can also be caused by clogged or dirty filter.
 
I'm pretty sure local tap water here is alkaline so most likely it's the decaying plants AND clogged filter causing it. The driftwood may also contribute to the problem but it's been in the tank for a long time so I will just leave it in for now. I already did a water change last night and clean up the filter, I'll check again this evening and see if it's improving. Otherwise, I may need to clean up the tank a bit more tonight. Thanks for the suggestions.
 
Alright, last night I tested the tank water and the kH is very low (only 1dkH). For the tap water the kH is 3. Now, GH is even lower...unless the test kit has gone bad (I bought it more than a year ago). This's kind of strange: if the tap water has such a low KH/GH how come it's alkaline?
In any case the soft water might explain why the pH swings. I looked at some of those buffering products in my LFS and they're all phosphate base. One even says it's not recommended for planted aquarium.
So, is there any 'natural' way to stablize my tank pH? I may try taking out the driftwood but my pleco loves it.
 
My tapwater is relatively soft also but comes out alkaline. If you are worried you could add baking soda at 1 tsp per 10 gallons to raise kH by 4 degrees. Do this slowly though to not stress your fish.

Other than fish keeping soft water is nice because I don't have to scrub soap skum or lime deposits away on my appliances and shower.
 
Seem like we found the cause. Baking soda can be good, but i prefer adding a bag of crushed coral the the filter or the tank. Acidic water will react against the coral and raise ph to a cartain 7.x level. When water reaches that pH, the reaction stops and pH stabilizes.

Choose one of the methods and go. Don't waste your money on pH raiser or stabilizers from the LFS. You'll get the same results from this method.
 
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