Any recommendations for keeping a slightly acidic PH community tank?

[Brett_Fishman]

I'm having a bit of trouble maintaining a slightly acidic PH in my tank. I'm trying for around 6.8ish, but it continously falls to about 6.0 (or lower).

First things first, our local water is about 7.0 PH and very low Hardness (basically rainwater). I have two pieces of wood, that i suspect are partially responsible for the PH lowering. It is a fully planted aquaria w/ CO2. Apparently, I've read CO2 can have an impact on PH. Is this true?

I've been slowly using PHiDJUST along with water changes every 3 days and it appears to be working. I read a few PH raising subtances that they are not plant safe, perhaps they contain high phosphates. My main concern is maintaining this PH while stilll keeping my plants healthy.

Any suggestions? How do you maintain your PH?

Thanks again for your help!

[TheOldSalt]

Oh, yes, CO2 definately lowers pH. We've used it for just that purpose probably before we tried growing plants with it, and some saltwater guys still use it to counter the effects of too much ozone, which can raise pH too high.
The thing is, though, that CO2 only drops the pH temporarily. However, if your water is very soft and has no buffering, it won't go back up when the CO2 dissipates.

Your wood is also dropping your pH, no question. Adding driftwood as a way to add tannic acid and drop the pH is a very old trick.

Holding the pH between 6 and 7 is pretty tricky, so don't feel bad. It's very common to have the water plummet down to 6 once it breaks the 7 barrier.
So what to do? Well, obviously, the solution is to increase the buffering capacity of your water, and there are plenty of things on the market you can use for that. There is even a product called Bullseye, which automatically sets your pH to the desired level of 6.5, 7.0, or 7.5, as I recall, by balancing the buffers just right. One easy way you could keep a pH of 6.5 would be to prepare some water ahead of time and store it for your water changes. (Adding Bullseye directly to a tank full of live fish & plants isn't a good idea. )
Otherwise you can just do it the little-by-little way, adding your buffer a little each day until it starts to hold your pH where you want it.

It's generally a lot easier to raise pH than to lower it, so consider yourself lucky.
As for any phosphate in your buffer, well, you can add more plants to contend with that, or read your labels to avoid phosphate, and while you're at it, reevaluate your CO2 needs and lower the dosage if it's too high.
The stuff you're using now...I don't know much about it. I'm sure that someone else does, and I'm likewise sure that major plant junkies like Simpte have to deal with this problem routinely, and can likely tell you something much more useful than anything I can.

[euRasian32]

i use a buffer to maintain 7.0. Seachem.

[DavidDoyle]

You are about to head down a slippery slope. ALtering water chemistry in a stable fashion is not an easy task. I would avoid most of the retail products mentioned above.

Running supplimental co2 will cause some pH swings in your tank however, they can be minimized. The key is in the kh.

Buffering Capacity (KH, Alkalinity)
Buffering capacity refers to water's ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the ``sponge'' absorbs the acid without changing the pH much. The ``sponge's'' capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added.
Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank's pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing). On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your fish, the buffering capacity makes it difficult to lower the pH to a more appropriate value. Naive attempts to change the pH of water usually fail because buffering effects are ignored.

In freshwater aquariums, most of water's buffering capacity is due to carbonates and bicarbonates. Thus, the terms ``carbonate hardness'' (KH), ``alkalinity'' and ``buffering capacity'' are used interchangeably. Although technically not the same things, they are equivalent in practice in the context of fishkeeping. Note: the term ``alkalinity'' should not be confused with the term ``alkaline''. Alkalinity refers to buffering, while alkaline refers to a solution that is a base (i.e., pH > 7).

How much buffering does your tank need? Most aquarium buffering capacity test kits actually measure KH. The larger the KH, the more resistant to pH changes your water will be. A tank's KH should be high enough to prevent large pH swings in your tank over time. If your KH is below roughly 4.5 dH, you should pay special attention to your tank's pH (e.g, test weekly, until you get a feel for how stable the pH is). This is ESPECIALLY important if you neglect to do frequent partial water changes. In particular, the nitrogen cycle creates a tendency for an established tank's pH to decrease over time. The exact amount of pH change depends on the quantity and rate of nitrates produced, as well as the KH. If your pH drops more than roughly two tenths of a point over a month, you should consider increasing the KH or performing partial water changes more frequently. KH doesn't affect fish directly, so there is no need to match fish species to a particular KH.

Hardening Your Water (Raising GH and/or KH)
The following measurements are approximate; use a test kit to verify you've achieved the intended results. Note that if your water is extremely soft to begin with (1 degree KH or less), you may get a drastic change in pH as the buffer is added.
To raise both GH and KH simultaneously, add calcium carbonate (CaCO3). 1/2 teaspoon per 100 liters of water will increase both the KH and GH by about 1-2 dH. Alternatively, add some sea shells, coral, limestone, marble chips, etc. to your filter.

To raise the KH without raising the GH, add sodium bicarbonate (NaHCO3), commonly known as baking soda. 1/2 teaspoon per 100 Liters raises the KH by about 1 dH. Sodium bicarbonate drives the pH towards an equilibrium value of 8.2.

from http://fins.actwin.com/aquariafaq.html

Note- you can add a bag of crushed coral to your filter in order to raise your kh. It disolves over time and needs to be replenished. If you add too much, it will also raise the pH a bit too.