How I stopped worrying and learned to love the mulm

This lovely photo is a rare look inside my sump tank. The reason I don’t normally show it is because it isn’t pretty. The fish tank drains into the sump by means of a solids lift overflow. This is a fancy name for a pipe that sucks from the bottom. As a result, the sump has got this brown gook on the bottom that looks very much like what you’d think. It’s mulm. Mulm is solid waste material such as plant parts, fish poo and leftover food in various stages of decomposition. Some of it gets pumped into the grow beds, but mostly it just lurks on the bottom of the tank. But wait, I can hear you cry, this is an aquaponics system – aren’t the plants supposed to clean all that up? Well, yes, sort of, but, as with most things biological – it’s complicated. I’ve posted previously about the nitrogen cycle and it’s a huge part of the process of converting fish waste to plant food, but it isn’t the whole story.

Nitrifying bacteria are delicate, slow growing chemoautotrophs which oxidize inorganic chemicals (ammonia and nitrite) to obtain their energy. The vast majority of bacteria species are heterotrophic and use organic compounds like proteins and sugars to fuel themselves. These guys are everywhere, as anyone who has ever found a sandwich from last year in a backpack on the first day of school knows very well (just me?). So the slightly more correct version of the aquaponic story is that fish eat food and excrete ammonia and poo, and heterotrophic bacteria break down the poo into organic and inorganic components such as ammonia, which feeds the nitrifying bacteria. Bacteria also feed on uneaten fish food, dead plant matter, dead algae, dead worms, dead protozoa and other dead bacteria. It’s a bacteria eat bacteria world in there.

Mulm builds up because larger solid particles take longer to break down than small ones. Eventually, it becomes mineralized. Mineralization is the decomposition of organic matter into its component inorganic parts. Some elements, such as copper, iron, manganese and zinc will not be available to plants until mulm is broken down, because they are only excreted in solid waste by fish. Mulm is also a generous source of calcium and phosphorous.

Another result of the decomposition of mulm is humic substances. These are somewhat mysterious molecules that give a yellowish or brownish cast to natural waters and are negatively charged. This negative charge binds with certain micronutrients and keeps them in solution and available to plants. They also prevent some metal toxicity in fish and plants.

Most of the preceding information I gleaned from an amazing book called Ecology of the Planted Aquarium, A Practical Manual and Scientific Treatise for the Home Aquarist by Diana Walstad. I’d recommend you find a copy, or google the author’s name and read her stuff online, because she has a fantastic understanding of aquatic ecosystems. In aquaponics, we are trying to create an ecosystem with terrestrial not aquatic plants, but much of her information is applicable.

Some folks filter out all solids, while others have a much more laid back approach. Systems using deep water culture, or floating raft methods need to filter. Red wiggler worms  help to minimize clogging by solids in media beds. I am of the opinion that letting bacteria do their job to extract all the nutrition from organic matter is beneficial to the plants and the whole system even if it looks a bit messy in the sump tank. No doubt if I was running a commercial system with high fish stocking rates I’d have a different position, but for me, for now, mulm happens.

I’m gonna eat some worms*

Red wigglers wiggling their way to the dark side.

My latest strange Kijiji adventure involved driving to some guy’s house, giving him some money and receiving a kilo of organic stuff in return. By far, most of the contents of the bag was vermicompost and worm bedding, but buried in there were a hundred or so red wigglers. Compost worms. Worms improve the overall health of the aquaponics system and require no extra feed or attention, so they are well worth the little outing to get them.

Before they can start doing their magic, they have to get from the baggie and into the grow beds, without bringing all their bedding and half-eaten food scraps with them. Thanks to the magic of YouTube, I was able to find a method that worked reasonably well. Just don’t tell the family that I used the salad spinner bowl to do it. Because worms really don’t like bright lights, they will automatically seek out dark spaces by heading down whenever they are exposed. I simply placed a handful of bedding into the strainer and let the worms find their own way into the grow beds. I did end up helping by plucking worms and placing them on the expanded clay media where they soon skedaddled farther down. I expect I won’t see them again until I pull plants out and find them among the roots. Worms can live very well in the media beds, even though they are periodically flooded, because the water is highly oxygenated and the worms breathe through their skin. They will adjust their population to match the amount of food available to them.

So what is it that they do in there? Contrary to popular belief, worms don’t actually consume decomposing vegetable matter as much as consume protozoa that live on decomposing vegetable matter. But this does have the effect of helping to break it down. What this means for an aquaponics media bed is that solid fish waste, uneaten food particles and sloughed off root material will all be broken down by the worms, which will reduce the amount of time I will have to spend cleaning the beds out. While they are doing this, they are also making worm castings – that magical stuff which gardeners wax poetic about. The worm castings are going straight into oxygenated water and being continuously circulated to the plants, which means the plants are being dosed with dilute worm tea at all times. Worm tea is reported to improve plant health by making micronutrients more available to the roots and making the plant more resistant to insects and disease. Apparently worm castings are also dosed with nitrifying bacteria, which, if you’ve been following along, you know are the bacteria that convert fish waste ammonia to nitrates that plants can use.

I’ve been running the system for a month, not long at all, but some of the seeds I planted are starting to look like real plants now. The ph is still too high, which can cause problems, and I’ll talk about what I’m doing about that next time.

*Remember that song: “Nobody loves me, everybody hates me, I’m gonna eat some worms!”? Yeah, I have that stuck in my head now. No actual worms were harmed in the making of this blog.

The tiny critters that make it all work

Source: Backyard Aquaponics

I’ve been saying that the aquaponics garden works because the plants use fish waste water as fertilizer and while that’s true, it is, like many things, a gross simplification. Fish poo and pee and also excrete ammonia directly through their gills. Not only is ammonia toxic to fish, but plants cannot actually use these things either. Luckily there are bacteria everywhere that will convert the ammonia to nitrates which plants do use. These bacteria may be everywhere, but it takes time before a sufficiently robust colony of them is established in an aquaponics system (or aquarium) so that a full stocking level of fish can be safely kept. The process of establishing this colony is called cycling.

Cycling can be done with fish in the tank or without, but I have chosen to cycle with a small number of fish. Madeleine has a healthy, fully cycled aquarium and she gave me some well used filter floss from her filter to jump start the cycling process. This stuff is loaded with the nitrifying bacteria that will colonize the grow media with its huge surface area and ultimately every surface in my system. I buried them in the grow media just under the water inlets of the grow beds. Nitrifying bacteria are pretty slow growing in bacterial terms, doubling in about 15 hours more or less, depending on temperature. The four small goldfish will hopefully provide enough ammonia to keep the bacteria fed while they are growing.

I’m doing daily water tests to determine whether the system is cycled yet or not. It can take up to six weeks to cycle from scratch. So far, I have not detected any nitrites or nitrates, and only barely detectable levels of ammonia. Since there will be a period of time before nitrates appear, I have added liquid seaweed to the system to provide nutrients to the plants, and a small amount of ammonia for the nitrifying bacteria. I’ve been heating the water in the tank to 21C, for the benefit of the bacteria, though the fish and plants are fine with it, too. Goldfish are able to tolerate a wide range of temperatures, a real advantage in terms of fish suitability for an aquaponic system, but for now they get to enjoy the warm bacteria-growing water.

Getting the system cycled is my immediate goal as it is the key to happy fish and plants. As an aquaponics gardener I am basically a bacteria farmer. If I can keep the bacteria happy, everything else should fall in place.