My Weekday Lab class this month is about diffusion and osmosis. While I was looking for everyday analogies to dazzle my students with, I came across one I had not expected to find or witness: the effect of diffusion on my piranha population.

Orinoco Piranha, Pygocentrus cariba

Orinoco Piranha, Pygocentrus cariba

Diffusion is a form of passive transport where molecules or particles, due to their own inherent motion, will move in a net direction down a concentration gradient from high to low until a dynamic equilibrium is reached.

Diffusion with Flow Arrows

Diffusion with Flow Arrows

Osmosis is actually a type of diffusion where the molecules or particles concerned are those of water. So that expression where you plan to “learn by osmosis” is a tad inaccurate unless you plan on dissolving your textbook in water.

So here I was, researching my topic and trying to come up with relevant examples of diffusion – meanwhile an example was staring me in the face. With several sets of eyes no less.

My temporary piranha tank, set up as a holding area until our piranha can be moved to their glorious, new 400 gallon tank at our Woodlands Xploration Station, was working well… swimmingly, as the saying goes. The nitrites and ammonia were nil, and the pH was stable at 7.3 (piranha are content with a range of 6.5-7.5). And while my pansy (yes, you read that right) piranha were skittish, they had been eating well and behaving normally. Then, the now-infamous Hurricane Ike came right up I-45 and left the Museum without power for 5 days. We followed our emergency plan and manually aerated our aquariums. The piranha pulled through wonderfully. But while we were counting our blessings, there was trouble brewing, seeping up from the deep, lurking in the most undisturbed of places… (gruesome huh?)

Yes, that is a piranha in our pre-Ike tank

While all of our permanent freshwater tanks have under-gravel filtration built in, the temporary tank did not. Several inches of gravel can hide lots of undesirable elements, even with the best filtration disturbing the surface. After power was restored, the piranha started behaving oddly, even moodily. They would only nibble at their food – and while that isn’t unusual for our piranha, several days of that in a row are definitely an indication that something isn’t quite right.

The movies would have us believe that piranhas are bloodthirsty, aggressive maniacs.  Picture the casino scene from 2006’s The Pink Panther where Inspector Clouseau (Steve Martin) leans on the edge of a piranha tank, and they proceed to shred the elbow of his jacket. In reality, piranha tend to be quite sensitive. Piranha have an excellent sense of smell, react to the presence of blood in the water and that will stimulate a fast feeding response. I have witnessed this “feeding frenzy” just a handful of times in the years of daily feedings, no pun intended. It has happened each time only when I introduced a new food type. After that they appear bored and once the tank leader starts feeding, they leisurely swim into the slowly sinking food and choose a morsel before leaving the area. It all seems surreal and calm considering the ideas many people have come to associate with a piranha feeding. It can also be frustrating if you are trying to make sure they are getting the amount of food they need and not letting it sink to the bottom and go to waste.

What I envisioned happening during Ike.

So after a few days of them just nibbling at their food and us coming back to scoop out the leftovers, we began testing the water. Oddly enough, our ammonia and nitrite levels were still negligible, but we did a partial water change in case there was food hidden somewhere.  After waiting a day to allow the tank to “settle,” a second round of testing included the pH, which the test kit indicated a pH level of  6.0. Our RO (reverse osmosis) water tested at 7.0 so we did another water change of only 30%, since rapid changes in pH are never good and piranha are very sensitive to quick adjustments (more so than our other fish with an under-gravel filter installed). No immediate response from the fish – and the next day, when we tested the water, the pH was still 6.0 or lower.

This continued through another water change and round of testing. Math is not my favorite subject but you’d think that if you had removed 1.5 times the volume of water in the tank (by the time all of the water changes had been done), the pH would have gone up at very least a little. But still no change, hence the puzzle part.

Here is the partial water change where we saw no net effect on the pH. Puzzling.

Eventually, we realized that the gravel could act as a sink for the hydrogen ions (from the breakdown of ammonia into nitrite)…And since our water changes did not disturb the bottom, the cause of our pH problem might be lurking in 3 inches of gravel. Here is where diffusion smacked me in the face.

In our next water change, we siphoned a small portion of the gravel as the water was removed (instead of just removing the water) and added our treated water back in. The water was crystal clear and the filter running for about 25 minutes when I got a call saying my fish were gasping - not dead, just in deep trouble. Panic set in and we raced to test the water: the pH had spiked to 6.6. Did I mention piranha were sensitive to rapid change? The fish were huddled near the output of the filter where the localized pH tested 6.3. A call to my local fish heroes identified the  problem (the disturbed gravel and the treated water pH of 7.6 – do the math) and the two-part solution: artificially lower the pH slowly, and cross your fingers.

What happened when we disturbed the gravel, releasing the trapped hydrogen into the water.

Two hours later, we had successfully restored the pH of the whole tank to a spicy 6.3 (higher than the original 6.0 but still outside of the ideal range.)  It seemed the piranha were adjusting, so this gave us a moderate game plan to gradually bring them back within their natural pH range. In the end, all of the piranha made it.

After we artificially lowered the pH, the piranha were not happy, but definitely not dead. Yea!

Lesson learned: the diffusion of particles of hydrogen from an area of high concentration in the gravel into a large tank of water with a significantly lower concentration occurs over time until an equilibrium is reached, independent of any other substance’s concentration. This will occur spontaneously and relatively slowly given the volume of water in the absence of other forces. The rate of diffusion will increase rapidly if energy is added to the system.

If the fish are acclimated to the lower concentration in the water; and the slow rate of diffusion that existed in the tank (from the gravel to the water) is rapidly increased, thus increasing the concentration of the water, there will exist a concentration gradient from the water to the fish.

Thus, your fish will go belly up as the hydrogen particles move into their systems decreasing the effectiveness of their gills (the gasping we witnessed). The removal of water from the tank and subsequent water testing a day later allowed the time needed for the diffusion of the particles from the gravel into the water to counteract the small water change. Once the tank had equalized, the net movement of particles was enough to account for the unchanging appearance of the pH. By disturbing the gravel, we added a significant force in addition to adding water with a higher pH.

Ah, science.

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