Some years back we lived on a small block back on the northern beaches of Sydney. The front yard of our house had the old septic tank sitting there, lid intact with an ugly pot on top as an attempt to hide it. Over the years prior to this, I had and maintained many fish tanks and ponds and felt it would make a great fish pond.
I removed the lid to find the tank was basically intact, just filled with plastic bags full of lawn clippings. We emptied the tank, filled the inlet and outlet with concrete mixed with waterproofing compound and we had a large deep pond. A filter system and a waterfall finished the job. Some plants were added in pots to protect the roots, a few Koi were installed and we had a water feature. The pond was filled from the roof from the house so water was no issue.
Today I would probably choose something other than Koi but it would still make a great start to an aquaponics set up. Aquaponics puts water from a fish tank or dam through a growing medium. We will have more on Aquaponics soon.
The pond attracted birds, frogs and all sorts of wildlife. All these creatures can eat the bugs that want to eat our veggies. They become our watchmen!
Not everyone has a left over septic tank, in fact we shouldn't have one either, so where do we start?
Fish need more than water volume, they also need surface area to exchange gasses, that is remove the CO2 and exchange it for oxygen. They also require water depth, if it is too shallow the pond will heat up far too much on hot summer days and kill the fish, likewise in winter it will potentially freeze through in winter if you like in a very cold climate. Finally a moving water surface is harder to freeze and will exchange more gasses. You are effectively increasing the surface area. Those bubble streams in fish tanks do the same thing. Making it deep also alows the fish safety from birds and cats.
So first, if you don't have a subterranean structure, lets dig it in. It will help keep the temperature more stable especially in summer. My Dad and I dug one into clay back in about 1980. 1.5 metres down, lined the floor with brick bats and mesh.. Then we laid a two course thick wall of bricks for the walls. The gap was from memory about an inch between courses. Then we laid a floor of 4 inches of cement with water proofing compound over the bricks and rendered up the walls with the same mix. The gap between the two wall was filled with the same mix.
Once the cement was set the gap behind the wall was filled with soil and gently tamped.
A week later we had a solid but very alkaline pond. We filled it with water, and it stayed full. Problem was nothing could live in that alkaline water, it needed to be aged or neutralised. I purchased some phosphoric acid and mixed a small amount in the pond. This sped the aging process and some almost 20 years later the pond still didn't leak.
While we were aging the pond a cap of sandstone was added around the edge as was two bowls to make a waterfall. A trench from the house for power to the pump that powered the waterfall was the final job. Two weeks later the pond was alive with goldfish water lilies and seedlings surrounding it.
Today there are many other options, ready made fibreglass, plastic liners, plastic ponds.
