A dripping, post-apocalyptic, Aldous Huxley nightmare…

This winter, albeit mild has been a hard one for the fish project! Here we talk about our main challenges chosen solutions and share photos:

IMG_1769In November, the tanks which we had maintained at 24 degrees during the dropping temperatures of late October were no longer holding that temperature over night and gradually in one of our tanks (which runs on overflow from another) we were seeing a drop towards 22… and then at its worst 19 degrees. The tilapia in that tank were sluggish and not feeding well and we could see that their immuno-response was reduced as every few days one would be lost to streptococcus – a naturally occurring bacteria that overwhelms fish when they are weak and causes haemorrhaging in the anus as shown in the photo. We decided to add more water to the polytunnel to increase thermal mass, in the hope that the heat we did add to the system wouldn’t be lost too rapidly. Then we further insulated the tanks with a rockwool layer fixed with pallet wrap, added mini polytunnels over the tanks, and we bubble wrapped the polytunnel – it worked – adding a few degrees to the tank temperatures, but was ugly and dripping – a post apocalyptic, Aldous Huxley nightmare…

The Polytunnel all wrapped up

The Polytunnel all wrapped up

With Aldous running, the winter embittered, and the air in the polytunnel was a night going down to zero, humidity was reaching 100% and the tunnel was literally raining inside. We had mould in the plant beds, and the less resistant plants were turning transparent where they stood, as the additional tenting prevented proper airflow.  So much water was steaming off the tanks that every week we were needing to add a few hundred litres to the system. Those water changes involved cold water from mains creating a quick dip in temperature, and made the fish pretty stressed. The polytunnel was ugly, wet and funky and the community supporters, fish and plants were grumpy and sagging.


We needed to make the space palatable again and fast. To combat this problem – which we knew would only get worse once we had NFT running, we realised we were going to need to heat the air in the polytunnel and reduce the heat difference between the tanks and the air. We considered a self-feeding rocket stove, or gas heaters, but because this project has volunteers coming and going, and also because we fairly regularly get vandalised, we finally opted for a 1000W space heater designed for greenhouses and humid environments. The heater hummed into action and we banished the plastic tents. The air temperature in the polytunnel, with the heater on a thermostat sits at 20 degrees, and the evaporation is markedly reduced. This has enabled us to lift the covers from the fish tanks, vastly increasing the natural light to the fish. It has also hage a massive effect on propagation – our little seedlings are practically taking off!

Now we’ve been building the NFT system for growing baby leaves salad – which looks like this –


And we now have pond weed in the sump tanks thanks to Dave who collected it from his Daughter’s allotment! – We plan on mixing this into the fish diet giving them a few handfuls a day once the weed is established.


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