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Performing a Winter Cool Down in Your Fish Tank

Many types of cold water fish will benefit and breed better, as well as show better colors, with a cool down period.

By Joshua Wiegert | February 10, 2012

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darter

Orangethroat darter. Photo credit: Derek Wheaton
shiners
Three orangethroat darters out of season. Photo credit: Derek Wheaton

FishChannelMaybe I'm weird - okay, I know I'm weird - but, winter is my favorite season, even though it is the season where most of nature is at rest. But, well, I grew up in a place where "summer" was the two week period you opened your windows in the evening. Winter is a very different season for our fish, in which their aquatic environment goes through some very obvious changes: it freezes.

Many fish (and other animals, as well as plants) use this season to their advantage, taking time to change the fat they put on all summer (and autumn) into other material. Most notably, many species of fish use winter to produce gametes and associated hormones. Female fish produce lots and lots of eggs, while male fish begin to produce secondary characteristics, including bright color, tubercules, and the like.

This winter "cool down" is actually required of many species of fish: without it, they simply won't be able to breed in the summer. This is true of many of our aquarium species, and many types of cool-water fish will benefit and breed better, as well as show better colors, with a cool down period.

The most striking examples of fish that require a cool-down period are the darters. Darters are a group of about 150 or so fishes native to the eastern half of the United States and Canada. They're probably one of the best known groups of North American native fish, though not often as an aquarium fish. They've often been the poster-child of aquatic species protections programs, as they're personable, colorful, and often endangered (though many are not). Many of them do make excellent aquarium fishes.

In early spring, shortly after the ice starts melting on ponds, you can travel to any good stream in the eastern United States and find these incredibly vibrant and colorful fish. Often times, you'll find one of these bottom dwelling torpedoes ardently defending his rock. It doesn't matter if it's another darter or something as large as a diver that comes too close, it is chased away from the rock - and believe me, I've snorkeled in cold streams and been attacked by darters.

When you stop to watch the fish, depending on species, you'll see an array of colors that challenge even the most incredible reef fishes for beauty. Depending on species of darter, you may be treated to a jet black fish with spangles of color around it on the drab end, or whirlwinds of rainbow red, blue, orange, and green on the colorful end. These are the males in the breeding garb.

The females are generally a lot less colorful, but even their colors come out in the early spring. They're also easily identified by their very plump bellies, full of roe.

Depending on the species of darter, spawning may occur over plants, under rocks, or even in small caves, though males will generally guard the spawn. In a few short weeks, the spawning activities end, the fry scatter, and the fish resume their normal activities, which does include a fair amount of territorialism, and some spawning. But, by the time the water temperature has reached the low 70s, the fish have lost most of their color.

This is mirrored in a lot of our native minnows and sunfish as well. If you've ever gone fishing in the early, early summer in a small pond, you've probably caught some breathtaking sunnies. While they're beautiful year-round, their color begins to fade as the water temperatures increase. Similarly, many native minnows show their best color after a cool-down period.

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darter

Orange throat darter. Photo credit: Derek Wheaton
darter
The brilliant red of the crescent shiner is characteristic of early spring, after a long winter. Note the grey bumps on the forehead, a secondary sexual characteristic of males. Photo by Joshua Wiegert

Among more familiar fish, the best example of a fish requiring a cool-down period to spawn is the weather loach. These fish are remarkably easy to spawn in the home aquarium. . . if you can cool them down for a bit first. Once the water temperatures increase, they'll spawn. They'll also take on a beautiful golden sheen that you'll not normally see in weather loaches. As an aside, they'll need to be mature before they spawn, which is about 6-8" long. The little 3-4" long ones you see in the aquarium store are not yet adults. The cool-down period, though, will also help them really grow.

A lot of our fishes are actually cool water fish, and may benefit from cyclical changes in temperature, including white clouds, rosy barbs, and the beautiful Bearded Corydoras (Scleromystax barbatus). However, there are certain caveats when giving your fish the cold shoulder.

No species of fish is able to tolerate wide changes over a short period of time; any changes made in temperature should be made slowly, by no more than a degree or two per day. Anything more is likely to really stress your fish, and could potentially cause ich or other disease.

The low temperature required to get most fish to start cooling down is generally only about 60 degrees, and the cool down period can be as short as two to four weeks. This can often be accomplished by placing the tank in a cold room, such as the basement or garage. It is generally within the scope of most chillers, though this is a bit pricey for my taste. In smaller apartments and homes where a basement or garage is not available, try this option instead:

Get one of those nice, square buckets that kitty litter often comes in. About three to four inches from the top, drill a hole to fit a bulkhead and place one into the hole. Put a lid on the bucket, and fill it with water. Put it on an outside windowsill, and secure it well. Run a small powerhead from your tank, out the window, into the bucket, and run a second line from the bucket back into the tank, via the bulkhead. You may want to cut some wood or even styrofoam to fit around the lines where the window is open to cut down on the draft, as well as possible insects, but the window only needs to be open enough to allow the plumbing to slip through. Ideally, this should be done in the spring or the fall when having the windows open isn't unpleasant - when the nights are in the 50s! With an aquarium heater, you can easily control the temperature of the tank. (I've also done this with cultures of live food in the summer. The bucket is covered with a fine screen that allows sunlight in, a bit of Daphnia is added, and a very slow powerhead put in. If you leave the screen cover off, you'll get mosquito larvae in there, which won't hurt anything and will probably get eaten before they hatch. 
During the cool down period, feeding should be kept to a minimum. Fish should be well conditioned before the cool down is attempted. 

In addition to lowering the temperature, fish will also benefit by decreasing the amount of light, mimicking the changes in daylight that accompany winter. After a few weeks of cool temperatures, begin to slowly raise the temperature. Again, no more than a degree or two per day. Also, increase the light period. Begin feeding quality foods again, including live and frozen foods. Your fish should begin to develop dramatic colors, and soon spawn.

So, winter is a very important season for our fish, too. Even though they don't appear to be doing anything, without their "cool down" period, they may be unable to spawn. If you're keeping cool water fish in the aquarium, try giving them a cool down this spring and see if it doesn't bring out some wild colors in them, and induce better spawns.

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Reader Comments

Dot    Hawk Junction, ON

9/12/2012 6:04:32 AM

Great ideas

alan    Ithaca, NY

2/16/2012 7:25:29 AM

Very interesting. If you can find thgem, check the 1950s issues of "The Aquarium" magazine. In those days, a standard practice was to collect wild fish like Banded Chaetodons and darters, in March, before the water has warmed. Usually the fishes spawned when the indoor temperature was right for them.

Autumn    Minneapolis, MN

2/11/2012 11:36:42 AM

Darters are really nice looking

Carl    Livermore, CA

2/11/2012 10:36:42 AM

Nice idea for a "chiller", thanks.

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