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Hi. I've got a 50 gallon fresh water tank -- fully established for going on 2 years. Not overstocked -- about 18 fish -- mixture of a couple of cichlids and a bunch of other community fish. Believe it or not, they all get along pretty well.

My pH and my nitrates, are driving me crazy. Here are today's stats from the tank water:

* pH = 5.0 -- too low
* Nitrite = 0.05 ppm -- not zero, but perfectly acceptable
* Nitrate = 0.50 mg/l -- way too high, I know
* Ammonia = 0.00 -- juuuuuust right

Now, last time I did a water change, the pH was also around 5.5 but restabilized at around 7 after I cleaned out the filters and did the usual once-a-month maintenance.

However, a few weeks later now and you'll note that the pH has again dropped significantly. The fish don't seem to be too bothered by it, but I know it's too acidic. My nitrates have historically been higher than I'd like, yet the fish don't seem to be too bothered by it; they are rarely below 0.25 mg/l.

BTW, my tap water pH, as measured today, is roughly 7.5. Today's water change was about 14 gallons, or roughly 1/3 of the available water, the rest being occupied by gravel, plants, rocks, etc. Plants are fake, BTW.

None of this has been a problem until about two months ago when the pH suddenly dropped. Nothing else has change materially.

The only thing I can think of is my gravel: It's heavy and compact -- ground up lava rock -- and I'm wondering if all else being equal, high nitrates and an inability to remove the gunk buried beneath deep in the gravel is contributing to driving down pH. Yes, I gravel vac -- once every six weeks or so, and when I do that I remove all the rocks and plants and stir things up really well. I usually do this a day or so before I then service the filter(s). Run two: a Fluval 304 and a Penguin 170 Biowheel.

Don't know enough about chemistry to know what this is all about -- but like I said, nothing much has changed, yet the pH falls.

Anyone know what gives? Thanks for any input.

Hi,
It sounds to me like you have a classic case of Old Tank
Syndrome...
Here is a link that will explain what it is, why it occurs,
and how to reverse the problem:

http://www.bestfish.com/oldtank.html

In the future, I would clean the substrate about a third
of the tank at a time. That gives the clean section time
to recover and the tank time to stabilize, before doing the
next section. The entire tank, once every 6 months is the
wrong interval.

Frank


-->>> The Confidence of Amateurs, is the Envy of Professionals <<<--

If you want to know more about aquarium chemistry, I'm going write a bit of an essay here:

Water chemistry is a complicated animal. Think of it like this. Imagine you have an empty glass pitcher. Water, like the pitcher, can have any number of things in it. Measuring pH is like asking what color the pitcher appears to be - it depends on what is in it.
The most important thing that effects pH is a class of chemicals I'll call buffers. Buffers are chemicals that tend to make the water acid or alkaline. Imagine you were to put colored marbles in our imaginary pitcher. If alkaline buffers are blue marbles, and acid buffers are yellow marbles, and we add both to the pitcher and look at it from a distance, what color would it be? It would probably be some shade of green, but the exact shade would depend on how many marbles of each color we had.
Now lets say that you had some very pure water (our pitcher has only 5 or ten marbles in it). You measure the pH and find it to be a bit low, so you decide to add something to raise the alkalinity. Whenever you add a chemical to the water, you are adding marbles to the pitcher, in this case blue ones. Adding only a little bit of buffer, say 20 marbles, will make the pitcher appear very, very blue. The pH has gone up a lot.
On the other hand, say you have water with a lot of chemicals in it (our pitcher has hundreds of marbles). If you add the same amount of buffer that you did in the last example, your 20 blue marbles aren't going to make much of a difference in the color.
In the aquarium, it is beneficial to have some buffers in your water so that day to day changes will not cause the pH to swing from one level to another. Large pH changes stress both fish and plants.

Many of the chemicals you'll here about when discussing aquaria effect pH. For example, nitrate and nitrite are acid buffers. The nitrate and nitrite levels are part of the nitrogen cycle. Basically, bacteria converts rotting wastes in your tank into nitrates in three steps - ammonia --&gt; nitrite --&gt; nitrate. Nitrate will build up, unless you do water changes (not just refilling evaporated water), and this can effect your pH. Again, however, how much depends on what else is in the water.
Other acid buffers include carbon dioxide, tannins (from wooden decorations), many phosphates (from fish wastes), and most other organic material.
Most rocks and minerals tend to be alkaline buffers. Limestone and seashells are two examples of calcium carbonate, a very common alkaline buffer. Baking soda (NaHCO3) is another.
On a side note, many but not all alkaline buffers are associated with either the hardness or the salinity of the water. Hardness is just a measure of the minerals present the water (whether they are buffers or not). Salinity is a measure of the salts present in the water (whether they are buffers or not).

The big picture, of course, a bit more complicated than a pitcher of marbles. First, some buffers have more longevity than others. Carbon dioxide is an acid buffer, and your fish breathe it into the water constantly. But, CO2 evaporates out of the water very quickly, and so doesn't build up in the water. Nitrates and minerals, on the other hand, will not evaporate. Because, the water itself evaporates slowly (the pitcher gets smaller) these chemicals are left behind and become more concentrated without water changes.
Second, some buffers are more potent than others. Adding an equal amount of baking soda and phosphate to pure water, for example, will consistantly give you a pH of around 6.5, even though baking soda is an alkaline buffer and phosphate is an acid buffer.

Now a few tidbits about aquarium remedies:
- Almost all fish are harmed far more by pH changes than they are by a pH that is outside their preferred range. Many plants are the same. If you decide to make any changes to the pH or hardness, do it slowly.
- All of the chemicals that you buy at your LFS to change the pH are just buffers. So, how much you need to add to have an effect depends on what is dissolved in your water already.
- Baking soda is an excellent alkaline buffer that is safe for fish and plants. 1 tsp baking soda per 25 gallons (5mL/100L) can be used daily until the alkalinity is raised to the desired level.
- Many acidity products are made from phosphates, which can cause algae blooms. Look for an acid buffer that advertises that it contains no phosphates.
- For a planted tank, one good acid buffer is CO2. DIY set-ups or pressurized bottles can be used to slowly bubble CO2 into the water, and thus raise the acidity.
- Peat moss does not always effectively lower the pH. It leaches tannins into the water and traps rotting organics, while at the same time absorping some of the minerals in the water. This raises the acidity while removing a small amount of alkalinity. Its tannin content and mineral absorption is limited, and so the effect is limited. If the change in buffering is significant enough, it could lower your pH.

The bottom line is that aquarium water is a complex mix of many chemicals. You can buy separate test kits that measure pH, alkalinity, acidity, hardness, salinity, and many others, and still not have a complete picture of what is in it. On the other hand, some aquarists have wonderful results with no testing at all. You can spend a fortune on pH adjusters and water conditioners, and many other products that adjust the aquarium environment. Yet, you can have great results without using any of them.
Ultimately, the best indicator of good water quality is your fish and plants. Spend some time watching them and enjoying them every day. That, after all, is what all of this is about!

Carl