By Stephen M. Meyer
Q. I have a tiny pond in my backyard here on Long Island New York. It is 18 inches deep, holds about 60 gallons and is home to seven 4-inch comet goldfish. Around Thanksgiving I remove the biological filter and do a large water change of about 40 gallons. (The filter consists of a 10-inch hexagon block of foam with a small pump attached to a fountain.) The fish have overwintered for two years without a problem (I open a hole in the ice daily).
The last weekend in March I did another large water change and put the filter back into the pond. Because the weather was warm I began feeding the fish once a day. The pH at this time was 7.2 and there was no trace of ammonia, nitrite or nitrate. It is now mid-May, the fish have been fed daily for the last eight weeks and the water has a slight hint of brown to it (no chemical filtration).
Now for my question. After eight weeks of water temperatures averaging in the upper 40s to mid 50s (Fahrenheit), the pH is still 7.2 and there is still no trace of ammonia, nitrite or nitrate. I can understand a functioning nitrogen cycle will leave no ammonia or nitrite, but where is the nitrate? The test kit I use is new and gives me readings when testing my aquariums, so we can rule out a defective kit.
A. This does indeed seem like a mystery. You are right, nitrogen — either in the form of ammonia, nitrite or nitrate — has to be somewhere. Let's do a back-of-the envelope calculation to see what the numbers should look like.
Your pond has seven 4-inch comets. Let's assume that each of these fish weighs about 10 grams. Under normal feeding conditions during the prime summer months each fish will produce about 2.5 milligrams (mg) of ammonia per day (250 mg per kilogram of mass).
During the spring, however, when feeding rates are lower than normal, water temperatures are still fairly low and the fish are rebuilding body mass, I would expect them produce maybe half that amount per day. So, seven comets would produce about 10 mg of ammonia per day.
A 60-gallon pond holds about 225 liters of water. Now, after 45 days the fish would have produced a total of 450 mg of ammonia. Assume for the moment that all of that ammonia was converted directly to nitrate and remained in the pond water. That would yield a total concentration of nitrate around 2 mg or l part per million (ppm). (Keep in mind that given all the uncertainties, I am rounding these numbers.)
Most of the nitrate test kits available to hobbyists measure in increments of 10 or 20 ppm. So, unless you spent big bucks for a professional nitrate test kit I doubt you would get an observable reading even if the nitrate level was substantially higher.
We also need to factor in the brownish tinge to the water, which is caused by species of planktonic algae. Those plants are consuming the ammonia before it is converted to nitrate. Therefore, it is probably the case that the actual nitrate concentrations are below 2 mg/l (per liter).
Then, too, you don't say whether there are water plants in your pond. If there are, they are certainly taking up ammonia and nitrate from the water. And your foam filter may have some anaerobic areas in it where denitrification is taking place. This releases some nitrogen into the air. Similarly, gravel or soil on the pond floor will act as a substrate for denitrifying bacteria.
The bottom line is, the nitrate is there but invisible to your test kit. More to the point, unless you have nitrate levels over 200 mg/l — which in a pond I think is virtually impossible — you have nothing to worry about and should not bother testing. Hope this clears up the mystery.
Digging Koi
Q. I have a 2-acre pond with a mud bottom. I heard that koi like to dig in the mud and could weaken the dam at one end of the pond. Is this anything I should worry about? What other fish could I keep in there besides usual game fish?
A. Koi, being carp, certainly like to scavenge the floor of their pond. They nose through the debris looking for things to eat and in doing so stir up the sediments on the bottom. The most severe consequence of this activity is to make the water murky. This nosing around may create such a mess that the koi make the pond uninhabitable for other species of fish that like pristine water conditions.
But koi really do not dig extensive excavations, and there really is no danger of them weakening the wall of your pond. In fact, many koi breeding ponds are built using an earthen dam.
I suggest you not add other species of fish to the pond, despite the temptation to do so. Mixing game species, tropical species and ornamental pond fish is asking for problems. Specifically, these animals all play host to different arrays of bacterial, viral, fungal and parasitic pathogens.
When you mix koi, bass, paradise fish, catfish or whatever, you greatly increase the risk of transmitting disease-causing organisms that some of the fish have no natural defenses against. The actual increase in risk is a function of several factors, not the least of which is the fish load in the pond.
A diversely stocked pond — even one well below my recommended maximum stocking density — that mixes coolwater, ornamental, game and tropical fish species is a very risky proposition. This is because the likelihood of casual contact among the different fish species carrying "exotic" pathogens is a virtual certainty. What I mean by "exotic" is a disease-causing organism for which some of the fish have no prior exposure. Those fish will be defenseless — their immune systems will not be able to cope with the pathogen.
Even worse, these infected fish may become a "breeding ground" for the pathogen, and population of this disease-causing organism could be large enough to overwhelm the immune defenses of fish that under normal circumstances would be able to cope. Once this kind of infestation mix gets going in a pond, it is impossible to treat. You stand a real chance of losing most of your animals...especially the koi.
No New Scales
Q. I'm a new subscriber to AFI. I have kept goldfish for about 10 years now. The ones I have right now, a comet and a "feeder," are about three and five years old respectively.
They have survived a number of diseases and they have probably run up against the filters intake tube a couple of times. Because of all of this, they have some "white" spots where I suppose that scales are just missing. They have good color and I hate to see these spots. I know that fish can regrow their scales. Why are my fish not regrowing theirs? Do I need to add calcium to the water?
A. Fish can indeed regrow scales, but invariably the new replacement scales are smaller in size than the lost scales, irregular in shape and "off color." The replacement scales may even be clear in color and therefore almost invisible to you unless you look closely. Sometimes the injury is such that the scales that grows back only partially cover the old wound site. Then, too, some wounds never regrow scales.
If months have gone by, then I suspect the spots you are seeing are going to remain permanently. Adding calcium to the water, or anything else for that matter, will not produce new scales at this point.