Saturday, July 19, 2014

A Wisconsin goldfish pond--tricks for pondkeeping

The goldfish pond around here used to get disgusting in mid-summer.  Several years of experiments have finally led to a method which works to keep the fish healthy and the pond not-disgusting.  Here's the rundown on goldfish-keeping which I wish I'd been able to find when we first set out to have a goldfish pond in our Wisconsin garden.

The "pond" is a 1700 gallon circular tank, made of some black plastic-type material--polyethylene, I think.  It is about the same size as the one at this link.  It was chosen to be too deep and too steep-sided for raccoons to get at the fish.  Raccoons do not jump in to catch fish, but rather, station themselves on the edge, or in very shallow water. In fact, our research indicated that shallower, or slope-sided pre-formed tanks sold to the unsuspecting as fish-ponds are known to landscape professionals as "raccoon dinner plates."

Another reason we chose a plastic tank is that, in Wisconsin, a concrete-sided pond would soon crack with our winter freeze-thaw cycle, while a metal tank might split its seams or rust if set into the ground.

Leveling the tank and setting it in the ground was quite a challenge.  The hole it needed was too narrow and too deep to dig with a bob-cat, so the hole was dug by two (quite wonderful!) garden helpers, A.H. and W.O., with shovels. Luckily, our soil is that deep with no rocks. (The weather is a caution, the mosquitoes enormous, but yes, gardening in the Midwest also comes with certain advantages!)

A.H and W.O. did a fantastic job of digging the tank in level.  After all these years of freeze-thaw cycles, there's still not even a quarter of an inch difference from side to side. Today, however, I believe I would call for a professional landscaper with a mini-excavator to dig a hole this big.

The tank is set about a foot above the ground level.  This leaves enough of the tank below ground-level so there is unfrozen water, no matter how cold it gets.  However, a thick layer of ice does form on top every year (I mean, this is Wisconsin...)

There is no gravel on the bottom--the tank is bare. This makes clean up much easier.

Around the protruding foot or so of the tank,  a few layers of limestone wall-stone, maybe 6 or so inches high has been dry-laid.  This matches exactly with the surrounding raised beds made of the same material. The rim of the tank is hung with limestone L-shaped veneer stone of the type mortared onto concrete-block chimneys and fences to make them look like they are made of field-stone. A local stone yard had these piled up (among the waist-high weeds and the wasp nests) in their returns/bargain area. The narrow end of the "L" is hooked over the rim, facing the water, the long end hangs nearly to the ground.  Arranged this way, the veneer covers the (quite narrow) edge of the plastic tank, and hangs just below the edge of the dry-laid limestone, so that the entire foot of black plastic sticking up above ground level is completely hidden with stone.

Our pond was originally stocked with 12 goldfish, bought from Wal-Mart for a few cents apiece. Not having any idea what we were doing, we bought fan-tails for pretty.  Later we read that fan-tails are not a good variety for outdoor ponds, because their tails are so fragile and hinder mobility.  Well, what can I say? Many of the fish have survived for years (this is their fourth summer) and they've grown and had babies, which are fan-tails too.  When we first saw the babies, we thought we were in for a population explosion but life outdoors is evidently hard: we now have 14 fish (although last summer, there were 16).

If we wanted to keep koi, I think we would have to be a lot more careful and particular, and that many of these tricks would not work. However, we are content with goldfish-keeping (and the fantails are truly beautiful fish!)

We installed the tank near an outdoor GFI outlet, as electrical filtration was our plan. Originally, we tried all kinds of filters, pressurized, in-pond, in ground, with built-in UV lights, with free-standing UV lights--you name it, we tried it, and boy was it expensive. We  also tried growing pond-plants as natural aerators.  Every combination of filter and light we tried, failed. The pond was scummy with algae, and the fish ate every part of every plant they could get at.  So, we gave up trying to filter the water, and gave up on plants, too.  Instead, we concentrated on aeration.

In our current system, air bubbles through the tank from a special "deep pond" aerator, which is hooked up, via a 12-line harness, to 12 airstones.  The airstones are scattered all over the pond bottom. The pond surface bubbles strongly when the aerator is on. Every year, I mean to buy weighted air-line, but I always forget.  In order that the air-line not float to the surface, the airstones and the harness are weighted with rocks.

The electrical outlet and the pump--which is not submersible-- are hidden under a "hollow rock" of the type designed to cover well-heads. The main trunk of the air-line is buried in the ground between the pump and pond-side.  At that point, the air-line emerges from the ground, goes over the edge and disappears into the depths.

We've tried many different kinds, and settled on Wardley's floating fish pellets. These seem to float long enough for the fish to eat them all, unlike flakes, which seem to start settling very quickly.

 To feed the fish, we turn off the aerator--the fish have learned this means food is to follow shortly, and they come to the surface as soon as the aerator is unplugged. After waiting for the water to become still, we feed the fish 2-4 times, 10 or so minutes apart, a table-spoon or two in each feeding, the number of feedings depending on whether the fish seem hungry for more. (Test=whether the fish rise to the surface and start "gobbling" when we come near.)  The 14 fish we have this year--the original survivors maybe 6" long and their various sized babies--cost several dollars a week in food.

After trying various combinations of UV lights, and even chemicals, we were at wits' end to control algae.  A visit to (the wonderful!!) Longwood Gardens gave us the solution.  The lily-pond patio featured a display of water plants with goldfish. The water was dyed a dark black with "pond colorant."  We adopted this approach for our fish pond--adding colorant to the water. The goldfish did not seem to mind.  We have since researched this issue and learned that goldfish are adapted to live in muddy water, which is also opaque.

On the downside, the initial expense was a bit high--$30.00.  On the upside, the jug we got, the only one at our local landscape supply, will probably last forever.  The stuff is highly concentrated, and packaged in quantities meant to color natural lakes. Another downside: we can no longer see the fish except when they come to the surface at feeding time. The water surface is utterly opaque. The best upside: no algae.  None.

Although we originally brought the fish into the house for the winter, the fish got too darn big--too big even to keep over the winter in a 70 gallon stock-tank.  With some trepidation, we started leaving them outside two winters ago, and they have now survived through two winters, including last year's never-ending polar-vortex event.

Once the weather gets very cold and the fish stop coming to the surface, we turn off the aerator and remove the airstone and line.  Before the surface freezes over, we drop in a gas diffuser, called a "pond breather."   This item does not keep a hole melted in the ice, rather, it draws water from below the freeze line and circulates it above the ice, then back down below the freeze line, allowing gas exchange to occur. It is designed to work with ice up to 15" thick.  Sometimes, the ice (with snow on top) gets thicker than that.  Then, the breather quits working for a while. However, even with our worst weather, this only lasts for a day or two.  As soon as the weather improves marginally, the breather starts up again.

In the spring, we wait until all the ice is gone, and the fish start coming to the surface. This is also when we start feeding the fish.  Two or so weeks later, when the fish are active all day long and eating regularly, all the fish are netted and put into a stock tank with an aquarium-sized aerator going--this is enough for the 1/2 day that the fish will be in the stock tank.  This is the only time of the year we get see the fish individually and close-up, to count them, to see who survived and how many new baby fish there are.

Once the fish are out, we pump the tank out using a submersible self-priming sump-pump designed to handle small solids. The water is pumped into a nearby bog garden.  With the water out, we clean the tank of all the leaves that fell in during the late fall and winter.  The fish-muck at the bottom we carefully dole out among the plants in the surrounding perennial gardens. Cleanup is a two person job, one to climb the ladder down into the tank (wearing hi-top rain boots) carrying a broom, snow-shovel and a pail, and another person to steady the ladder and receive the muck (swept with the broom into the shovel, then transferred from the shovel to the pail) which remains after the sump-pump has gotten out all it can.

At this time, we also re-lay the airline and the airstones.  Most of the airstones can be reused, but the air-line has to be completely replaced with new.

The tank is then refilled with city water from the hose-end.  It is conditioned with whatever de-chlorinator chemicals the local big-box store is carrying.  We also add barley extract.  We put the fish directly into this fresh new water. They seem very happy to go--not at all discombobulated from the change of water.

Despite the aeration and the lack of algae due to the colorant, the pond water gets thick with particulates in the summer.  The fish are large and active, and eat a lot of fish-food. So, several times a summer, we pump out 1/2 the water and refill with fresh hose water.  The pumped out water (with all its fishy nutrients!) goes to water the plants in perennial gardens and the bog garden. When we start running fresh water in with a hose, we also add the de-chlorinator chemical--the chemical is added over maybe a couple of minutes to the stream of running water.

The water is pumped out once in June, twice in July and twice in August--that's the theory, anyhow.

A bird net is arched over the tank in fall, in an attempt to keep leaves out of the water. The net is held off the surface with PVC water-supply pipe.  This pipe, which is very flexible, is made into arches by slipping it over 18" long rebar pounded in around the pond-edge. The 1" square bird-netting is laid over these arches, the L-shaped veneer stone hanging from the tank-edge weights the edges to keep it taut.

There are no small children around here, and no legal requirement to fence a decorative fish pond. We thought that installing the pond with a good foot or so sticking out of the ground, and the water surface so far above the ground surface, would prevent anyone falling in. This worked well in general, until one day, a friend came over with her dog. The dog jumped in and paddled around very happily, but was unable to get out again. That dog is now a constant visitor, so we put up a chicken-wire fence--ugly, ugly, ugly. I have to admit that fencing is one part of the pond-keeping art that we still have to figure out.  We are cogitating on it, though, so hopefully,  there will be another post (in a year or four) saying what we did to get rid of the chicken wire.

It certainly was expensive to try out all the different filter systems.   Also, it something of a disappointment that there are no plants: the original vision was to have water plants in the pond, but the fish were voracious and ate every plant we tried, from water lilies to floating plants, to little floating plant-islands.

Yet, despite the plant-less nature of the pond, I think it does add an element of liveliness with its mysterious bubbling black surface, and the occasional flashes of gold as the fish come to the surface.  At feeding time, the fish are plainly visible at the surface, and having the pond does add a focus to garden walks. In the end, although it was very expensive to figure it all out, and although the pond isn't quite what I first imagined, I do think the answer has to be "yes," it is worth having a pond.


  1. Can you tell me where you found your large fish pond tank?

  2. Regretfully, it was so long ago, I cannot remember. If you should find one, please write back and let me know, because others ask me this question also.