Four Fundamental Facts of (Aquatic) Life

[plebian]

The following list consists of facts essential to understanding aquariums and fish-keeping. Afterward I provide an example as an illustration. References are provided at the end of post.​

1. Aquatic ecosystems are extremely complex and only partially understood.1
2. Aquatic ecosystems are dynamic.2 The species that inhabit them must be adaptable.
3. The mechanisms of adaptation are genetic and behavioral, each of which effects the other in a kind of feedback loop.3
4. Both genetic and behavioral variation exist among the members of any given species. This is commonly referred to as intraspecific variation.4

The first fact in that list includes microbiomes, both host-based and free-living, that include hundreds if not thousands of inter-dependent microbial species. Of these, nitrifying bacteria are best known among aquarists. However, nitrifying bacteria alone are not sufficient for sustaining a healthy aquatic ecosystem.

The topic of microbiomes will not be addressed here. I mention it to bring attention to a topic that is almost completely overlooked by aquarium hobbyists and stimulate further individual research.

The following example will illustrate the last three facts on that list.

Example:

The Amazon Basin is frequently described as having three very different river systems characterized by their water chemistry. The three types are commonly called blackwater, whitewater, and clearwater.

Blackwater rivers are characterized by low pH (3.5-6), low nutrient content, low mineral content, and very clear water except during the rainy season.

Whitewater rivers are characterized by moderate pH (6.3-7), high nutrient content and very turbid waters with low visibility.

Clearwater rivers are similar to blackwater rivers but more variable. Nutrient levels vary from low to moderate, while pH can range from 4.5 to 8.0

One of the most popular fresh water aquarium species, Discus have historically been collected from blackwater rivers and lakes. However, Discus are known to inhabit all three river types, depending on geographical location and seasonal changes in water flow.

Nevertheless, almost every reference you come across on the Internet will insist that Discus are a blackwater fish that must be kept in water that is very soft and acidic. This is simply not true.

Most references you find on the Internet emphasize the importance of pH and and dGH (water hardness) in general when describing various species of tropical fish and their requirements. In fact, the opposite is true. The pH and hardness of water should be of least concern to most fish keepers. There are very few species of fish that cannot tolerate a wide range of variation in either.

The following is a quote from a technical document, PH Requirements of Freshwater Aquatic Life, a summary of current research produced by RBI, a water resources research company:

“With few exceptions, pH values between 6.5 and 9.0 are satisfactory, on a long term basis, for fish and other freshwater aquatic life.”

In a reference to fish farming, the authors go on to state that “for maximum productivity the pH should be maintained between 6.5 and 8.5.”

The key word here is “maintained”. The actual pH value of aquarium water is not very important. What’s important is that it remain relatively stable. Rapid fluctuations can lead to problems. The same is true for dGH. In fact, very soft water is more likely to lead to problems than very hard water.

Since this post is not meant to be a scholarly article, I’m not going to expound on water chemistry any further, other than to say it’s complicated, as is everything else in nature.

To the point at hand, hard water and pH should only be of concern to those fish keepers interested in breeding. Some species won’t breed in hard water with a high pH and vice versa. It’s also true that the eggs of some soft water species won’t develop properly due to calcification in water that’s too hard. The effect on juvenile and adult fish, however, is inconsequential.

I’m going to finish this exposition by describing my personal experience with pH and water hardness.

I keep Discus, Tetras, Corydoras, and two other species of catfish in water with a pH of 8.0 and water hardness of 250 ppm (14 dGH). All of my fish, which are species native to the Amazon River Basin, are healthy and active. The Discus have spawned multiple times (though none of the eggs have survived). The Diamond Tetras have spawned repeatedly, producing healthy fry every single time.

The Diamond and Cardinal Tetras, and the Corydoras, are all wild caught.

I know an importer that specializes in the less common varieties of South American catfish. Almost all of his stock is wild caught, including Discus and Altum Angelfish. He stopped using RO filters to produce soft water because they are extremely wasteful and, as he learned, unnecessary. He now relies on city tap water with a pH between 7.0-8.0 in all of his aquariums. Those aquariums contain more than 50 species of fish from Brazil, Peru, Venezuela, and Columbia. He has experienced no problems with any of the species he keeps.

References:

1. Microbiome Composition and Function in Aquatic Vertebrates: Small Organisms Making Big Impacts on Aquatic Animal Health.

2. Water chemistry of the Amazon basin: The distribution of chemical elements among freshwaters

3. Mechanisms of species divergence through visual adaptation and sexual selection: Perspectives from a cichlid model system

4. The ecological importance of intraspecific variation

 

[2tank]

very interesting, I have the opposite issue where my tap water is about 5-6.5ph, I used to spend lots of time trying to fix my ph and with KH salts and stuff. but int he end everything seems to do better just letting it go on its way.

 

[GaryE]

There is wonderful stuff here, but there's always a "but". Extreme habitats produce extreme variation. You have fallen into the over-generalization trap, I'm afraid. The very complexity you talk about also occurs in fish, as well as in water. The following statement is false, and you actually prove it to be a mistake:

"Most references you find on the Internet emphasize the importance of pH and and dGH (water hardness) in general when describing various species of tropical fish and their requirements. In fact, the opposite is true. The pH and hardness of water should be of least concern to most fish keepers. There are very few species of fish that cannot tolerate a wide range of variation in either."

Let's look at requirements. If they are to breathe. eat and swim, you're good. Mere, basic captive survival? You're right. Most softwater fish will adapt to harder water. You would kill hardwater evolved fish like many Poecilia with your approach, but you didn't go there. But what is being a successful aquarist?

A fish, like a human, is a process. With fish we go from sperm and egg, to fertilized egg, to larvae, to fry and onwards. For a great number of Amazon basin species, like other softwater region fish, eggs cannot develop outside of a narrow range. There is debate whether this is because sperm aren't viable in harder water, or whether eggs take in too many minerals since they are evolved to grab what they can greedily, or both, but it remains that your Discus have told you something. In a healthy place, their eggs would have been viable. Eggs are part of the process of the fish. If they can't develop, then the environment is not right for the species.

I learned this with Apistogramma gibbiceps. I worked with an importer, so I knew where my fish came from. We received gibbiceps from a blackwater habitat. and I took some home to see how they worked. They spawned many times in my pH7.4, 140ppm water over several years, and the eggs were never viable. Eventually, they died of old age. That was not successful fishkeeping. But they came in again, caught in a whitewater habitat. They were identical to my previous group, to the human eye. This time, they produced small broods of viable young, again and again. They were easy to breed, though the majority of eggs didn't develop properly. I got my consistent dozen fry per go.

Breeding west-central African species, I was able to radically increase brood size by softening the water. It's a long established breeder's trick, known for many decades. Very soft water is a difficult environment, and our fish are often evolved (there are generalist species too) to exploit very narrow niches. It's a serious issue with habitat change and climate change, as the same adaptation that allow some species to thrive in extreme homes will hasten their destruction as humans change those habitats. Specialization is a double edge sword.

Your Discus say you're a little off base there, as much as everything else you wrote makes perfect sense. If we as a hobby are to be consumers - buyers of fish as living ornaments, you are correct. If we want to keep fish to observe natural behaviours, or to propagate and share them out, you've really overgeneralized. Eggs are organisms too.

 

[plebian]

Discus have told you something

Not anything I didn't already know. Did my Tetras also tell me something? Selective observation is a common fault among those attempting to defend preconceived beliefs. I didn't overgeneralize anything. I stated quite clearly some of the exceptions, in particular with egg development.

As for eggs being life, they are also food. I don't want my Discus breeding because I have no outlet for the fish they would produce. I don't want any of my fish breeding. I actually removed all of the male Tetras (so I thought) to stop it. Uncontrolled breeding is simply irresponsible.

Some "purists" in the hobby insist, sometimes vehemently, that you must replicate the natural habitat as closely as possible. If that were so, then they should also be advocating for the introduction of predators and parasites into hobbyist aquariums.

 

[GaryE]

Diamond tetras are one of the easiest tetras... I know you already know that.

Some "purists" in the hobby insist, sometimes vehemently, that you must replicate the natural habitat as closely as possible. If that were so, then they should also be advocating for the introduction of predators and parasites into hobbyist aquariums.

That is a bit of a straw man argument, is it not? There is a large difference between advocating respect for the natural history of a species, and introducing Hoplias and fish lice to the tetra tank. "As possible" is a key phrase!

Anyway, welcome to the forum! Enjoy.