So my opinion on the importance of pH in the discus aquarium has changed. After three years of watching these fish closely, and being lucky enough to have a GHL aquarium products Profilux 4 I’ve seen pH change in real time. And I’ve observed no effects or changes to the behavior of the fish at all. And pH has been 7.5 down to 4. And they were fine and still are fine.

As I understand it, pH is mostly dependent on your KH (Carbonate Hardness). If you monitor your KH and you have a reasonably steady value, it’s very unlikely you’ll see a significant change in your pH. With a KH of say 4, your pH will likely fluctuate around 0.5 pH over the course of the day. If you have plants, your pH will change as your lights come on and go off. The photosynthetic activity of your plants will change as light levels increase and decrease and this will vary your pH over the course of the day. pH will increase while your lights are on and decrease as they go off. Plants will add O2 and use CO2 differently throughout the day. As the CO2 goes up the pH comes down and so on.

So pH is never constant. With a traditional test kit this may not be apparent and most keepers probably assume their pH is constant if you tested at the same time every day.

Your filter bacteria consume KH. Your fish produce acid in their waste. So your KH will continue to decrease until the next water change replenishes the value. KH is carbonates and bicarbonates in your water. So your water change water needs a suitable amount to reset your KH value in your tank to continue to buffer your pH value. Carbonates will neutralize acids until they are used up, thus keeping the pH stable. If you are replacing water with RO, you can constantly run your tank with a low KH, but you will see your pH going up and down constantly, in some cases by considerable amounts.

I’ve watched over the course of the week the pH go from 6.7 to 4.2. When some KH containing water is added on the water change the pH will increase back to 6.7 in a matter of minutes. The fish are not at all bothered by this change. All had excellent colours and some were spawning. The tank has done this for many months. I continually read and I’m told this is bad. But I have never seen any ill effect on the fish or their behaviour. I assume this happens in the Amazon, a rain forest, where the river is subject to regular heavy rainfall, which runs into the river and must surely change the pH. The scale is obviously massive compared to a fish tank but the principles are the same.

The pH crash

I really believe the only issue with pH is if it gets so extreme that your filter bacteria is affected. This will be affected before your fish (but I wouldn’t want to test this). If the filter bacteria die, ammonia is produced. Your fish really don’t do well with even a hint of ammonia. So it’s the reduced pH that crashes the filter that kills the fish. Having said that I have never let the pH go below 4, but I’ve never seen any fish distress at a pH of this level. But the risk of disaster is much higher as you’re getting quite close to wiping out your filters.

So I think it’s best to have a nice KH buffer to keep you away from the low pH disaster zone. There’s no benefits of having such a low pH (although bacteria and parasites won’t like it), and it’s quite stressful not really knowing when disaster is about to happen. As said the disaster is the filter crashing, not the effects of the low pH on the fish.

Get it Stable

The best solution I have found that has given me an almost constant pH of 6.3 is the following. I say almost constant because it varies throughout the day as said. But the variance is quite small, around 0.3 pH.

I’m using RO water with a KH of around 3. So the tank KH can start to dip quite quickly. The solution to that is to add a small amount of coral sand to your tank. This is basically Calcium Carbonate that slowly dissolves helping maintain the KH levels. I added around a pint but it will depend on your tank. This has helped keep the pH stable for quite some time, but I presume I will need to add more at some point.

Aeration

The other trick and some may not be able to do this one as it relies on the GHL Profilux 4 is using the aeration to help stabilize the pH. I always had constant aeration running. But the constant up and down of the pH over the course of the day made me think what would happen if the air only came on when the pH started to drop. Surely I could flatten the pH line if the aeration came on at certain times. The aeration will force the CO2 out of the water reducing the acidity. So if my pH drops 0.3 below the set value, the aeration comes on until it’s back to 6.3.

I now have a pH of 6.3 almost every time I look at the gauge. The daily variation in pH is now minimal. And the fish do not seem to need constant aeration.

So having said pH doesn’t matter, I’ve done all this to keep pH constant. And it’s worked very well. But I no longer have the constant worry that my pH is constantly dropping and at some point will kill my filter bacteria if I don’t add new water.

I’m currently only changing 20% of the water, once per week. pH will be 6.3 + or – 0.3, guaranteed.

A word of caution: the pH 4 swings described above are far outside normal guidance, and I only monitor them this closely with continuous automated dosing and testing. If you’re new to this, aim for the steadier setup described in the Get it Stable and Controlling pH sections rather than letting pH swing this low, since a crash in filter bacteria can kill a tank quickly.