Adventures with Our New Tosai Pond

My husband Ryan and I dove headfirst into the koi hobby in 2014. We laid out a garden hose, kicked the hose out larger and larger, then my husband got our old 9N Ford tractor and pushed out dirt as deep as he could…not deep enough. We rented a small Bobcat and still didn’t get the job done. So, we hand-dug all the sides of our 50-foot by 26-foot, 11,000-gallon koi pond. We told ourselves we would never hand-dig a pond again!

We Have a New Koi Pond, Now What?

Now we had a koi pond, but we knew nothing about koi keeping! We joined the Central California Koi Society (CCKS) and members donated koi to start us in the hobby. We were grateful to receive free “pond fish” because this was a low-risk way to experience the trials and errors of koi keeping.

But we couldn’t escape the hand digging as we expanded our pond system to 13,500 gallons a little bit at a time. We added two skimmers, a large settling tank, bakke shower (designed and built by Ryan), large Japanese mat bio filtration system, turtle pond, quarantine tank, etc.

Why Build a Tosai Pond?

Over the years our koi keeping was improving, but the quality of our koi had not because of spawning in our pond and not knowing how to select koi. It was time to improve the quality of our koi but we did not want to spend thousands. Along with very knowledgeable friends in CCKS, we made frequent visits to koi dealers and started learning more about choosing quality tosai (first-year) koi with “potential.” We increased the quality of our koi food too and had some success growing tosai into nice koi. But some of our CCKS friends still had better success with growing tosai because they had them isolated in a separate tank.

A tosai tank allows similar sized koi to grow at a faster rate because they are not competing for koi food with larger koi. It also allows us to save on koi food costs by feeding our tosai high quality food and feeding koi in our main pond less expensive food.     

The Tosai Pond Project Begins

Fast forward to May 2024, and Ryan has a week vacation with no real plans. So we jump into adding a tosai pond to our pond system. Now it may seem we make impulsive decisions, but we talk and “dream” about our projects for months or years before we pull the trigger. We discuss and debate our ideas for design, function, cost…then debate some more. We research, talk to our friends at the Central California Koi Society (CCKS), and watch videos online. Then we put our dreams into action when an opportunity arises and just do it!

We had a small, red-ear slider turtle pond that didn’t function well. The water flowed through our pond system, but it always had algae and seemed too stagnant. It was about 2.5 feet deep and 6 feet in diameter…another whim project we built in a couple days a few years ago. We decided to convert the turtle pond into a tosai pond for our young koi between 1-3 years old.

We initially wanted the tosai pond to be 5 feet deep and 10 feet in diameter but scaled it back to 4 feet deep and 9 feet in diameter thanks to the hard pan and huge jump in liner price for 10 feet in diameter. How hard could it be to dig out another 1.5 feet down and 3 feet wide by hand? Digging in clay is not easy, but manageable. However, hoisting the heavy clay above your head out of a 4-foot-deep hole and into a cart in the 100° F Central Valley sun is the hardest thing I’ve ever done!

Why-o-why did we decide to do this? Well, I guess it is because we are koi kichi and get huge satisfaction from our hand-built, home-designed, low-cost  projects! We have saved thousands of dollars over the years…that we apparently just reinvest in buying more koi!!

Ryan added an aerated bottom drain to the tosai pond and plumbed it into our pond system with 4” pipe with valves so water flows in from the bakke shower, through the Japanese matte bio filter, then into the tosai pond. Then water flows through the bottom drain and into the waterfall to the main pond. From the main pond, water goes into a large settling tank, through bead filters, then into the bakke shower, etc. We can restrict or bypass water flow into the tosai pond and the water turns over about 4 times per hour.

We completed the tosai pond with less than $2,000 (so far…still need to add flagstone decking) and in little over a week expanding our pond system to 15,000 gallons!

We waited about a week to ensure the system was working properly. Built a PVC pipe cover with 30% UV sunshade to protect the koi from sunburn and predators. We then moved our 7 tosai from our quarantine tank and added a few nisai we acquired as tosai last year. We added an auto feeder and went to bed feeling like our koi were safe and protected.

Tragedy in the Tosai Pond

Early the next morning, my husband found two of our favorite koi – why is it always our favorite – had jumped out. Our very nice budo goromo tosai and our largest karashigoi nisai were dead…we were devastated. We went through all this effort to give our koi a better environment and we lost our two favorite koi. There were no signs of predators nor anything wrong with the water quality, we think the new environment must have spooked them and the cover didn’t work as intended. So we modified the net and kept close watch on the rest of the tosai, so far all is well.

Despite the rough start, we are still very happy with our tosai pond! We have a new place to sit and watch our beautiful, higher quality koi. We hope in the long run that our tosai pond will produce some top-quality koi and potentially win some koi show awards.