The following is a murder-mystery involving ornamental pond fish and a periodic lethal environmental disease. You are witness to these events, in first person, as they unfold:
"Hey Bob, have you seen the Manual to the DREL-1?" (Hach Spectrophotometric water quality assay kit), it was my partner, Rick, foraging around.
"No; but it should be downstairs in the carrying case. Last time I saw it was when I gave the pH/water quality pitch at the Koi Club of San Diego".
R: "Aha!; then you must have lost it!
B: "No way, eh."
R: "Well, help me find it will you?"
B: "Okay, but tell me what you want to test. I've used the Hach kits (they're identical to most of the "pillow-reagent" kits in the aquarium hobby) so many times, I may know how to do the tests you have in mind without the manual.
R: "OK, here's the scenario: I just got an urgent, make that extreme emergency, call from one of our service customers (X); they've lost and are losing a few tens of thousands of dollars of imported Japanese ornamental koi carp (nishikigoi) right now! I'm trying to get the gear together and get out there hyaku (pronto in Japanese), ascertain the cause(s) of mortality, and rescue what fish we can."
Quickly I reviewed what I knew regarding the system in question: about ten thousand gallons capacity, built a year or so ago, adequate depth, shade, no prominent, previous problems. The pond, falls and related life-support had largely been designed, built and entirely managed by our companies. Most of the fish (gulp) were on loan to customer X by an important business associate from, you guessed it, the Land of the Rising Sun. So I asked:
B: "Rick; what did the people on site say the fish look like that are dead and dying?"
R: "The dead ones are laying on the bottom stiff and some have a slight bulging of the eyes (emphasis mine). The others, that are still living, are a mixed bag. Some are swimming listlessly, disoriented, some seem to be gasping at the surface".
B: What else or different has been done with the fish of the system in recent time?"
R: "Here's the account folder."
Rick and I look over the Aquatic Life Services account History Forms for customer X. ALS had been there some four days earlier, done some routine testing, physical cleaning, partial water change, checked the mechanicals, but nothing else, and no record of observations that anything was amiss.
R: "What do you think?"
B: "At first, I imagined the problem might likely be poisoning, due to the location of the system being in the middle of a huge wholesale/retail plant nursery. You know, the rapid onset, disorientation; but tell me; what size fish died/are dying first?"
R: "There are no fish under eight inches; the larger fishes, over fifteen inches are doing the worst...".
B: "and are any of the fish jumping, "flashing", skittish; I mean agitated-animated? Are any showing dark spots or blotching?
R: "No to both".
B: "Well, usually, acute organophosphate toxicities involve these and other indicative symptoms. Are you sure nothing else has been done with the system?"
R: "Actually, yes. One of the on-site managers mentioned the main (two horsepower) filter and recirculating pump had been switched to bypass the filter for a few days by accident & asked if it'd cause any harm to just rotate the valving back to the filter. I told him to backwash the pressurized sand filter first and that otherwise it should be fine. As you know, we installed a reverse-flow bio-filter in the uppermost basin, under a bridge, but I didn't think that this might be a problem due to anaerobic decomposition because of flow from the auxiliary pump flow."
B: " What is the main filter pump?"
R: "A (Sta-rite) Dura-glas two horse..."
B: Why a Dura-glas? Surely they'd prefer to pay the slight additional initial cost for an upper-line Max-e-Glas series motor (considering the better service value- greater quietness, less electrical cost...)
R: "Oh, they wanted a two-speed motor to run on low for normal operation and high for backwash and for when people they want to impress come over."
B: "Oh no! Tell me, after back-washing the filter; was the pump left to run on the higher speed?"
R: "Yeah, as a matter of fact, they had asked if it'd be ok to do so to remove a slight cloudiness that we thought was due to bypassing the sand filter a few days. I didn't think over-driving the sand filter would be a big deal because the bio-filter is of such a large volume and surface area after it. So, what do you think the problem is?"
B: "Differential gas pressure. Whose out there now?"
R: "Robbie; he's removing the dead koi and making a massive water change, as we assumed the problem was some sort of acute chemical or physical toxicity."
B: "Please beep him and make sure he does not turn the pumps down or off, and leave the fish in the pond (carp have an amazing capacity for "rising from the dead"). Let's get our act together and buzz out there." I grabbed some other test gear including our YSI (Yellow Springs Instrument) combination electronic dissolved oxygen, pH, temperature and conductivity meter. We drove out, calibrated and dropped in the YSI probe and quickly assessed the situation.
B: "Wow!, look at the reading for dissolved oxygen concentration; 10.6 mg/l at twenty three degrees Centigrade!"
R: "Way beyond saturation; water at this temperature should hold a maximum of 7.0 ppm."
B: "Uh huh, an look at the pond, you can actually see bubbles coalescing and rising to the surface."
R: "Robbie's got something he wants to show you".
B: "Jeez, look at the bubbles formed under the skin of this fish! Ouch! This must be excruciatingly painful; talk about the bends!"
R: "So what do you think happened here Sherlock?"
B: "What is happening here is emphysematosis, gas bubble disease. This is an adverse condition brought on by too sudden a change in the pressure of dissolved gases inside the fish and the water they're in. Let's go down to the pump room."
"Aha Watson!" Just as I suspected. On switching the main pump to high speed you can witness air being entrained. See the bubbles there, through the clear trap cover? What else? I bet you if I lean on the plumbing slightly, hmm, hmm. Hear the air suction noise?
R: Holy coca cola! Let me guess; the air is driven into solution by way of the increased pressure around the impeller, elevating gas saturation in the water, increasing gas concentration in the koi..."
B: And then through rapid decrease in water gas pressure by the water rapidly warming and the pump being switched to low speed..."
R: "The bends!"
B: "That's a right. The difference between the gas pressure being suddenly higher inside the fish versus outside the fish..."
R: "Like popping the top off a bottle of soda on a warm day..."
B: "Gross. Well, Rick, what do you think we should do?"
R: "Bring the concentration of dissolved gas back up so the fish's bodies can out-gas; just like in a decompression chamber."
B: "You know this "environmental disease/adverse condition" is a lot more common than thought and easy to prevent. In this case, if the pump had been securely mounted to the floor to reduce vibration and the plumbing to it had been fit with a schedule 80 nipple and routinely checked for developing air leaks, I don't think we would have had this problem."
R: GBD (fancy acronym for gas bubble disease) is sometimes caused by the interaction of too much sunlight, fertilizer and plant/algae metabolic activity. Pretty much the same gas pressure differential problem but by biological causes.
B: "And in aquaria you can have similar difficulties from the use of too-fine (usually wooden) airstones with too many fine air bubbles. This is one of the reasons for bilateral exophthalmia ("pop-eye" on both sides) as the eyes of fishes are highly vascularized."
R: "Lots of blood vessels." "I should have known, Sherlock. Many aquaculture facilities, public aquaria and well-engineered pond systems utilize a gas-dissipation system; baffles, water falls and other de-pressurizing systems to release "extra" super-saturated gases."
B: "Other symptoms include gas bubbles in the blood and lymphatic fluid, in the body and fins. Strongly affected fish have frayed fins and a hard time "keeping their balance."
R: "So, aquaria and pond hobbyists should keep in mind for avoid GBD you need to provide continuous aeration, prevent excessive photosynthesis by plants and algae, and be aware that mechanical systems can entrap and supersaturate air into their systems. Even that water temperature differences and water changes with tap water that may be highly supersaturated can be GBD deadly."
B: "Correct: gas bubble disease is caused by a sudden drop in gas pressure in the water. Fishes that have been living in that water can't rapidly adjust/get rid of the gas in their bodies and consequently suffer from gaseous embolism.
Amlacher, Erwin. 1970. Textbook of Fish Diseases. T.F.H. Publ., Inc. N.J.
Fenner, B. and Rick Aspray. 1983. Ornamental Fish Ponds, Filters: Design, Construction and Maintenance, Freshwater and Marine Aquarium. 6:6.
P'enzes, B. and I. Tolg. 1983. Goldfish and Ornamental Carp. Eugen Ulma GmbH & Co., Germany.
Sniezsko, Stan. 1976. Diseases of Fishes; Environmental Stress and Fish Diseases. Tropical Fish Hobbyist, N.J.