Last year we embarked on a trial of aquaponics in a domestic setting, as part of a PhD undertaken by John Grant of Sheffield Hallam University. Our related blogs are not a guide in themselves, but perhaps help fill some of the gaps we found in existing literature. If you want an introductory guide we recommend Sylvia Bernstein’s Aquaponic Gardening.

This blog follows up on our previous post with a review of our first 6 weeks of hosting fish, and the snags we had with our initial build. There was a gap of some months as having built the physical system we were nervous of the temperatures that could be reached in our polytunnel and so waited until the summer so we could monitor temperatures before introducing fish to the system.

The build

Our first visit to the polytunnel after the winter break was a bit of a shock, as the foundations for the water tank had subsided. This meant we had to empty the water tank and dig out the earth around the sump tank to the extent that we could embed boards to stop the earth from collapsing against the tank.

Lesson learnt: if placing the fish tank over an IBC sump tank, box in the sump tank – however firm you think the ground is. We’d also suggest that the top water tank is filled some time before adding fish, if you are not building on a very firm base. The fix for our problem would have been a lot harder if we had already put fish into the top tank, and very disheartening if cycling had already started.

Having plumbed the growing beds’ inlets and outlets we added water to check the autosipons worked. It is worth doing this without the clay balls in place. We also discovered a leak in one of the tanks at this point.

Lesson learnt: It is much easier to fix these things before the clay balls are added.

Down the track we have realised that it will be difficult for us to alter the height of the outlet pipe that sits up in the tank. Ours is a single piece of pipe down to the junction with the outlet pipe that returns to the sump tank.

Lesson learnt: Plumb the system so that the pipe within the tank can be removed.

We have also realised that whilst we take water from close to the top of the fish tank, this can still carry a fair amount of gunk (technical term). It is likely that this affects the water flow.

Lesson learnt: Install a settling tank in the plumbing immediately after the fish tank.

Which fish?

Originally we had hoped to stock Tilapia or trout, but were concerned about the need to heat or cool the tanks. Having reviewed various guides it became clear that carp were one of the most robust fish when it comes to temperature range and fluctuations. And, as we have a lake full of carp on our doorstep, carp it was. We don’t normally eat carp because it is very muddy when fished from the lake, but we realised that keeping it in the aquaponics fish tank offers a way of cleaning out their systems,and perhaps make them more appetising.

We spent a significant amount of time trying to source organic fish food but it seems to be available solely to large scale fish farms. The first batch was a floating feed from a local ‘World of Water‘, which we have followed up with a slow sinking feed for coarse fish from Skretting. We are also considering how we can farm duckfeed as a high-protein green for them.

Lesson learnt: We recommend a floating feed. Whilst carp are, naturally, bottom-feeders, they were happy eating from the surface after 1-2 days, and this gives you a chance to check on them.

Whilst considering the animal kingdom, it is worth noting that we found a frog in the sump tank one day. It is worth ensuring that the tank is sealed, and/or providing a route out for anything that decides to go for a swim.

Cycling the system

We used the Murray- Hallam cycling technique to start the system off. This means adding liquid seaweed and adding plants, and then waiting for 2 weeks before adding fish. There was concern about the pH of the water, which was higher than the ‘ideal’ range given in the literature. However, having tested the pH of the lake from which the fish would be taken, we realised they were the same, so no action was taken on its acidity.  This has dropped over time.

We were also able to introduce some rocks from the lake into the growing beds. They carry ‘healthy bacteria’ that would help speed up the cycling process.

3 solar powered oxygenators were used over the summer, which are now powered by the mains as daylight hours start to reduce. We cannot currently monitor oxygen levels in the water, but hope to get a more efficient approach to the energy consumption in future.

The rationale for this approach was that the water would be prepared for the fish. However, 2 weeks into their residence we found a number dead. This hit us hard as the whole idea of this approach had been to avoid stressing the fish. It appears to have happened because the stocking levels were too high, and they received more food than necessary due to some enthusiastic helpers. We reduced the fish levels to a quarter of that suggested in the literature, and their feed is also given at a lower rate than suggested. A month on the pH, Ammonia, Nitrite and Nitrate levels remain spot on.

Lesson learnt: Use the Murray-Hallam method but introduce the fish very gradually, and give a single person responsibility for feeding. It is better to lose some early plants than put the fish through stress. 

Data monitoring

We are yet to start the formal monitoring of the system but readings were taken daily during cycling and the first two weeks of the fish residency. Now the system is up to speed we are testing pH, ammonia, nitrite and nitrate levels on a weekly basis. We found (but don’t know if this is the norm) that the Nitrite level rose and fell very quickly, and the Nitrate level rose to 80ppm on the day after the fish were introduced and haven’t fallen much since.

We also visually check the water levels and flow rates in the tanks and from the outlets into the grow beds. We have found that we have to add around 300 litres of water on a fortnightly basis over the summer, suggesting that evaporation is more of an issue than we expected.

Our greatest concern is the temperature, as this is very difficult to control, and there is no data available on this for our particular site (a polytunnel in the UK). The good news is that the water appears to limit temperature swings with its highest temperatures on the hottest days reaching 5 degrees less than the outside air, and staying 5 degrees warmer than the outside air overnight. It is likely that the water will provide a similar cushioning effect over winter, and it is hoped we will have full temperature monitoring to provide more information to protect our fish, and help others with their aquaponics plans.

Output!

First plants go in, and visibly struggle to find nutrients

The planting of the grow beds was circumstantial rather than planned. A tomato cutting or two have taken hold surprisingly quickly and are now fruiting. Some supermarket coriander plants thrived for around 6 weeks but didn’t survive the first cold night of the autumn, whilst some strawberry plants surprisingly fruited within weeks. Spare pea seedlings grew, but failed to fruit. And of the greens, the lettuces have taken well but various brassicas have been eaten by unseen bugs (or slugs?).

At this point it seems that the output is different to that achieved in soil. Some leaves, like spinach and mizuna, are growing well and deliver on taste and texture; but other lettuces are just too limp. Next year we will need to more consciously trial what works and what doesn’t.

And finally, perhaps most exciting for a beginner, that thing where people just scatter seeds into the grow beds – it works! In early August, in a rush before going on holiday, I sprinkled a handful of dwarf peas into the bed and covered it with a single layer of clay balls. Those plants are now growing nicely and producing flowers. Light levels may now be too low to deliver many peas this year, but they may well survive the winter ready to grow on in the Spring.