How Low Can You Go?
Low ph and aquarium fishes
- Randy Carey -

All rights reserved by the author
Randy's Aquaria

Recently a group of five scientists wanted to measure the affects of very low pH on some Tilapia (Oreochromis mossambicus). Over a six hours period the pH was carefully dropped to 4.0. Once the pH reached this low reading, guess how long it took before the scientists observed [1] abnormal behavior, and [2] the first death:

  1. 30 minutes
  2. 3 hours
  3. 3 days
  4. 1 week
  5. 3 weeks
  6. no observable difference after 1 month

While you are thinking about your answers, keep in mind that Tilapia are indigenous to east Africa. The Baensch Atlas claims it comes from running, standing, and even brackish water. Recommended pH is in the 7's.

Iíll return to the Tilapia experiment and the answer in a little bit.

Experiences in low pH

Recently I have been keeping and spawning fish in what many consider very low pH. I didnít have success with Apistogramma panduro until I gave a pair their own tank and a pH of 5.0.

When I first noticed a group of Scissortail Rasbora (Rasbora trilineata) going through pre-spawning activity, I checked the pH and found it had dropped to below 5. After that tankís water dropped to 4.4, I observed pre-spawning activity of my Yellow-Tailed Congo Tetras (Hemigrammopetersius caudalis). I successfully spawned the Rasbora in a pH around 5.0 and Iíve obtained eggs three times from the Congoís with a pH in the mid 4's.

I've watched a pair Rasbora macualata spawn in peaty 4.4, and three times obtained eggs of the dwarf African tetra Neolebias ansorgi (and a successful hatch), also with a pH targeted at 4.4 .

A dwarf cichlid which is a challenge to spawn is Spadetailed Checkerboard (Dicrossus maculatus). After months of no luck, Iíve been getting spawns with very soft water having pH readings in the 4's and 5's.

Recently, a fellow aquarist noticed that one of my maculatus females was guarding fry. After scanning the bare bottom 20 gallon tank (intended for holding, not spawning), I observed that of four females, two had fry and one had eggs. I quickly checked the parameters. The water was mostly all r/o and the pH had dropped to 3.7 !

Going back to the African tetra Neolebias ansorgi... I was housing six adults in a 5 gallon tank. Again, the water was mostly r/o. I had not changed the water for sometime when I checked the pH. My electric meter read 2.9 ! The fish showed no signs of stress!

A reasonable skepticism is that my pH meter was in error and needed to be calibrated. I thought that. I tested the meter against a control liquid of 4.0 . Yes, the meter was off a bit . . . by a whole one hundredth! The 4.00 solution read 4.01 . The African tetras were indeed living in 2.9 !

Not all fishes have been stress-free when my pH falls. Usually the cichlids are the first to show stress, but sometimes I notice a difference between tetra species. So far Iíve noticed the groups best tolerating (enjoying) low pH are Rasbora, African tetras, and some Lebiasinids (the Pencilfishes and "splash tetras").

The biotopes

One reason these fishes tolerate low pH must be attributed to the water parameters of their biotopes.

In the past decade, German authors have been publishing water parameters form various biotopes. Linke provides readings for Southeast Asia and West Africa in his book Labyrinth Fish.[1] Here are the pH readings for the licorice gouramiís (genus Parosphromenus): 5.4, 4.8, 5.5, 4.3, 4.5, 5.5, 4.8, and 4.1. Not all readings in Southeast Asia have pH this low, but some habitats do. And some fish groups thrive in these waters.
[1] Linke, Labyrinth Fish, the Bubble-Nest-Builders, 1991, Tetra Press

In the same book, some African readings are given. I found readings of 5.7, 5.6, 5.7, 4.8, and 4.8. All of these sites had a general hardness of 1 or less.

Then there is South America. Linke's book {2] Dwarf Cichlids covers this continent as well. I found several readings in the 5's. Four readings (all Rio Negro locations) all had 4.3 with general hardness less than 1.

[2] Linke & Staeck, American Cichlids I: Dwarf Cichlids, 1994, Tetra Press]

Earlier I mentioned that I found dwarf cichlids did not tolerate as low pH as did the tetras. Since this book discusses dwarf cichlids, I assume that it ignored some South American habitats with lower pH because these waters do not hold Apistogramma. In other words, South American could quite plausibly have streams or pools with pH reading in the 3's.

As we look at readings of various biotopes, we find habitats all over the tropics with pH readings in the low 4's through mid 5's and with hardness at 1 or less.


Back to the Tilapia...
Scientific experiment

Five scientist [3] from the Netherlands set up 6 tanks. Each 25-gallon tank housed 7 Tilapia at 79 degrees. The fish were isolated from human contact except for two healthy feedings per day. The fish were housed in pH of 7.4 for six weeks. Then three of the tanks were "gradually" lowered to a pH of 4.0 in a six hour period. The other three tanks were used as a control.

[3] van Ginneken, van Eersel, Balm, Nieveen, Thillart, "Tilapia are able to withstand long-term exposure to low environmental pH, judged by their energy status, ionic balance and plasma cortisol," Journal of Fish Biology (1997) 51:795-806.]

Remember my opening question?

Once the pH reached 4.0, how long do you think it took for [1] the first fish to show signs of abnormal behavior, and [2] the first fish to die?

The answer is that at day 37 the experiment ended with no fish showing any signs of abnormal behavior. No fish came even close to dying.

The twice a day feedings continued and the amount of food consumed never dropped. Tests were conducted on the blood, its sodium and chloride content, adrenaline, muscle tissue, and other technical readings. If the fish are stressed, the readings from these tests will show it. However, no statistical differences were noted between the fish from the acidified tanks and those from the control tanks.

These results defy what most aquarists expect.

Most of us have lost fish as the pH "goes South"--and often this is well before it reaches 4.0. How did these fish survive such a rapid, drastic, and prolonged drop of pH?

Those conducting the Tilapia experiment were very careful to emphasize that when they acidified the water, all other parameters were kept the same. Furthermore, the fish were not subjected to stress conditions. I believe this is valuable information of which the aquarist should take note.

First, the experiment made sure to maintain the "ionic balance" of the acidified tanks. Ionic balance refers to the presence and ratios of important ions. These ions seem to be calcium, magnesium, potassium, sodium, sulfate, and chloride. These are the key parameters in the German-based book by Mayland/Bork

[4] Mayland & Bork, South American Dwarf Cichlids, 1997, Landbuch-Verlag, Hanover, Germany.] and in a June ë98 TFH article by Joe Gargasn [5, Gargas, Exposing pHallacies, part 2, Tropical Fish Hobbyist, June 1998].

In American Cichlids I: Dwarf Cichlids, Linke and Staeck list these parameters to show the differences of clear, white, and black waters.

The experimenters listed the chemicals they used to obtain this ionic balance: magnesium chloride, potassium chloride, calcium chloride, NaHCO3, and sodium chloride (regular table salt). This is amazingly close to the ingredients list that Joe Gargas gave me for obtaining ionic balance.

Another factor emphasized by the scientists was the absence (or immeasurable presence) of Aluminum ions. Apparently its presence causes stress in fish.

Lastly, extra care was taken to avoid stressful conditions. The fish were not netted and had already settled into their tanks for 6 weeks. Ample room was provided. The temperature was reasonable and constant. The only human contact they had was the twice a day feedings.

The authors of the Tilapia experiment acknowledge that previous research by others demonstrated different results: Fish subjected to very low pH would show "general impairment."

But our experimenters also point to an earlier paper (Balm & Pottinger í93)[6] which proposed and demonstrated that acidification can be tolerated if [1] the reduction of pH is gradual and [2] stress from handling or from aluminum ions is avoided.

[6] Balm & Pottinger, "Acclamation of Rainbow Trout to low environmental pH does not involve an activation of pituitary-interrenal axis, but evokes adjustments in branchial structure," Canadian Journal of Fisheries and Aquatic Sciences (1993) 50:2532-41] The results of the Tilapia experiment furthers this claim.


How important is pH?

This article has been discussing low pH as it applies to aquarium keeping. Ironically, the lesson to be learned is that pH might not be as all-important as we have come to think. Sure it is important, but not at the exclusion of all other parameters. I believe conductivity and ionic balance will start to receive more attention in the coming years.

Perhaps pH is just a general, but not always complete, indicator of what is going on with the waterís chemistry.

If a fish spawns or dies in low pH, perhaps the cause is not the pH but something associated with the low pH. For instance, the spawn might have been induced by low conductivity which is associated with low pH. Or a high waste content, also associated with low pH, could be the real cause of a fish's death.

The physiology of fish is a very complicated matter. So is water chemistry. We aquarists have generalized the water parameters to pH, dH, and temperature. A fish and its water involves a lot more than that.

I reported that I spawned several species in a pH below 5.0. I do not feel that it was necessarily the low pH that triggered the spawn. I suspect that much of the success comes from low ionic content (low conductivity) which is often associated with low pH.

In any case, many fish can tolerate pH levels lower than what we aquarists have come to think. Certainly, many come from habitats with readings in the 4's and 5's. Such information can be found in recent aquarium literature.

So keep your pH meter. Check regularly. Know the water parameters of your fishís natural habitat. Do frequent water changes. But, depending on the species youíre keeping, donít panic if the reading falls into the 5ís. Your fish might prefer it. That is, if the 5ís arenít too high for them ;-)


All rights reserved by the author
- Randy Carey -
Randy's Aquaria