With the onset of cold weather, the temperatures in my unheated greenhouse fell into the teens this last week.  The few remaining plants were not active nor numerous enough to cleanse the fish water and the cost of keeping the water in the temperature range tolerable to these tropical fish (and the bacteria that support the system) could no longer be justified.  So… this first trial season was over.  The process of decomissioning the system was a bit tricky so I’ll share the experience. TWIMC. . . 

Due to an earlier operator error (forgetting to turn the circulation pump and aerators back on) there were just 11 fish remaining – most quite small.  Few, if any, are large enough to fillet, but still needed to be retrieved from the subterranean tank.  To keep my options open regarding the cooking or further raising of the fish, I made an effort to match the water temperatures between the new container and the old one to reduce stress.  I also put them into new water to cleanse their system for improved flavor.  In this case it meant using well water with some of the iron precipitated out combined with the recommended level of aquarium salts.  I purchased a fancy fish net with a blunt end and fine mesh suited to their diminutive size for the extraction itself.  New data perhaps, but the fish people say the tilapia can tolerate water temperatures down to 61F.  Mine tolerated 54F, though its probably not recommended. 

The fancy new net for extracting the fish with the buried fish tank and insulated cover in the background.

 I transferred the aquarium heaters to a 5 gallon bucket of the clean water and brought the temperature up to the same temperature as the water in the buried fish tank. It took patience but I was able to catch the fish (and didn’t fall in).  They are an unusual blue color appropriate for their name: Blue Nile Tilapia.  One of the down sides of having the fish tank buried is the limited light available for fish gazing.  The size and number of fish remaining was also unknown until the “harvest”.

My goal was to drain everything into the fish tank to preserve the water for watering potted plants in the greenhouse and perhaps as a “seed” for next season’s water and bacteria.  The subterranean tank will be partially heated by the earth and partially by a single aquarium heater or livestock tank heater over the winter – or so the plans goes.

Two of the aptly named Blue Nile Tilapia.

The circulation pattern of the water in the system is from the fish tank to the syphon bed, then the middle tank, and finally the raft bed before returning to the fish tank.  I worked in reverse order and started by shutting all the valves between the tanks. I was able to syphon out the raft bed down to about 1″ by filling a hose with water at the hydrant, then carefully placing the “uphill” end under the surface of the raft bed water then dropping the other end into the fish tank. When the syphon failed due to low water I took out the last bit with a wet/dry shop vac until there was not a connection of water across the bed to freeze and push out the walls.  That water was not saved.  The middle tank was straight forward using the same sort of syphon system.  

The last of the raft bed water was removed with a wet/dry shop cac.

Drawing a syphon up the syphon housing

The greatest challenge came with the syphon bed. There was about 2″ of water remaining and the only way to access it was down the opened up syphon housing.  I finally decided to use a hose ball-valve on the “uphill” side to enable me to keep the water in the hose (primed). So I had to hold the other end up – pinched against the bed – while I opened the valve under the level of the water and then I could get the other “downhill” end to the destination tank and release it.  I was able to evacuate all but about 1/4″ of water.  I tried to further shop vac the rest out to no avail.  I’m hoping the pressure from that much ice will not be enough to damage the bed construction and that the hydroton pellets will somehow interrupt the ice formation.  Time will tell.   

I will be monitoring the water temperature in the buried tank to see how it tracks as winter progresses.  Note to self – maybe start this a little earlier next year.  The shortened daylight ran out on me two days in succession during this process and having wet hands (or other parts) in freezing temperatures is not pleasant.