Aquaculture Greenhouse: The Sand Filter

I recently found out that a new coworker is something of an expert on setting up large aquariums and their filtration systems. This is something of a boon to me, since I don’t know a lot about it and am currently doing lots of research to fill the cavernous gaps in my knowledge. I only got to talk to Mike for about 5 minutes on the subject, but he already found a big flaw in my plan. Fortunately, I think it is one I can fix.

As I mentioned in a previous post, I am planning on flooding the lower two feet of my future greenhouse and growing fish under the path between beds. The goal is to create a balanced aquatic ecosystem. Ideally, the organic compounds that leach out of the soil or come from the greywater will be enough to feed the phytoplankton in the water, which will then become the base of the food chain in my tank.

The problem Mike found is in my soil. My garden beds, which will extend 3’ above the water level, will be made entirely of organic matter, with minor amounts of sand and pea gravel thrown in for soil structure. Organic matter that is below the water line can be a problem, though, as it will undergo anaerobic decomposition and pollute the water with toxic chemicals. My solution was to fill the area under the soil with sand.

According to Mike, any sort of filter for biologically alive water will quickly fill with a sort of slime. It is a good thing, as it really helps filter the water. However, it will grow to fill the pore spaces to the point where no water will get through and the water will back up and drown my plants. The only way to stop this from happening is to stir the sand every couple of months to break it up. This is obviously not an option as the sand will be underneath 3’ of productive garden soil.

My first thought was to come up with some sort of tray system. The weight of the soil above makes horizontal trays too cumbersome to deal with, so I thought vertical trays every few feet might solve that problem. That still requires a structure around it to enable me to pull it out and there are lots of ways that could go wrong.

Another option would be to just put drains every few feet just above water level. That way, when it plugs, there is an overflow drain that would continue to function. This is still a very viable option, but I really don’t like it much. When the sand is completely plugged, it becomes not only unusable space under the soil, but also stagnant and anaerobic. I am not fond of that plan.

Then it occurred to me that the primary purpose of the sand was not as a filter, but as a spacer, keeping organic soil above the water line. What if I used something else as a spacer instead. The soil will still do most of the filtering necessary, so the sand isn’t really needed. The product that comes to mind most quickly is a system of grids that are used for rainwater retention systems. They are made of a stiff plastic and are designed to hold a lot of weight while still having a lot of open volume for water storage. If I could put those in there and a layer of landscape fabric on the top, I would have something that would most likely not plug and would increase the amount of water I could hold in there considerably. Of course, there are other structural features I could use. I could go all Roman aqueduct on it and use stone arches. That would be pretty cool and would last forever. It would probably be cheaper as well. Stacked cinderblocks would probably also work, but wouldn’t leave as much room for water.

So far, the open water solution is my best. It increases the volume of water for the fish to swim in from 1100 gallons to around 2500 gallons. But that leaves me with a new dilemma. What organisms do I allow to access that area. It will be very difficult to get in there to do any kind of maintenance, so anything that dies in there stays in there. Also, if I decide that a particular organism is a pest, I will really never be able to clean it out of the system without draining the pond and starting over. The cavern also gives the fish a good hiding place, which may or may not be a good thing.

I love this process.

Aquatic Systems of Life

Ever since I was a kid, my hobby has been creating terrestrial systems of life. The trick with creating good ones is to balance the cycling of nutrients through the system such that the beneficial organisms out compete the disease-causing organisms. In terrestrial systems, you have a plant to cycle water and oxygen and provide a centerpiece. In larger systems it can also provide food. You have a fungus to decompose decaying plant material. I prefer mushrooms because they also provide food, even in small systems. Earthworms till the soil and decompose vegetative and fungal matter. Beneficial bacteria take up most of the rest of the niches. In order to keep the system healthy, a regular infusion of organic matter is about all that is required. Without the mushrooms and earthworms, disease and pest organisms, such as gnats and disease-causing bacteria and fungus can get in and dominate the system, reducing the health.

After my big idea of creating a stock tank with tilapia in it a few months ago, I am considering a foray into the creation of aquatic systems of life. First of all, let me say that I don't know a lot about it. Yet. I do need to find two good books. The first needs to be on aquaculture. The second will need to be on how to create and balance aquatic systems of life. If any of you have any good suggestions, I would certainly appreciate them.

The basic need of aquatic systems is going to be the same as in terrestrial systems. The flow of nutrients through the system is going to be crucial. However, in aquatic systems, the balancing of this system is much more important than it is in terrestrial systems. Water is a much more robust system for life to flourish in, so algal or bacterial blooms that could potentially be harmful can happen in a matter of  days rather than weeks like they would in soil. First of all, fungal allies are out of the picture, so other players need to be introduced to do a lot of the decomposition quick enough that the harmful bacteria don't get to it.

Right now, I am thinking that the first step to creating the pond at the bottom of the greenhouse will be to create a robust system with players that are either really tough or pretty easily disposable. For the first year or so, I will be actively building soil in the garden beds above the pond. This will involve the addition of lots of organic matter that will need to decompose into healthy soil. As the grey water from the house filters through the decomposing organic matter and down into the pond, it will carry a lot of nutrients with it. I need organisms in there that will filter out those nutrients in a way that is healthy for the aquatic ecosystem.

Those organisms can balance the system until the tilapia arrive, with many of them providing food for the tilapia once they get there. I am sure that the cast of players will change as I do further research and give it more thought, but here is the cast of characters I am currently considering:

1) Water from a real pond - I will find a healthy pond and collect a gallon or so of the water from it. Ideally, it will be a clear pond without obvious signs of pollution or algae blooms or the like. I think I have just the source. This water will carry with it the necessary bacteria, algae, and other microorganisms for treating my water and making it healthy. It will also form the base level of the aquatic food chain, the food chain that will feed off of the nutrients in the water and feed my larger forms of life.

2) Snails - I need to be careful about this one, but I am thinking that aquatic snails will serve as grand decomposers, helping to fill the niche that earthworms take up in soil. They will burrow through and consume the layer of muck that ends up on the bottom of the pond. I just need to make sure that I find a species that is aquatic only and won’t climb the walls at night and eat my plants.

3) Duckweed. From what I understand, duckweed will meet two major needs. It will filter nutrients from the water, cleaning the water. It will also serve as an eventual food source for the tilapia. It reproduces rapidly, so a solid colony of duckweed in place when the tilapia arrive would be a very good thing. There are probably other aquatic plants that I could put in to serve a similar purpose.

4) Crawdads (crayfish) - They are not native to my area, but are very common in the lakes and streams around here. As such, local wildlife managers want rid of them as much as possible, so I have a nearly endless supply. Crawdads will also help consume dead and dying matter, especially animal matter, like dead fish. I am hoping that they will also act as a food source for the tilapia. I’ll have to look into that one. The one note of caution is that crawdads eat fish eggs. Too many, and my tilapia won’t be able to reproduce. That is, unless they can eat the crawdads. It might be a self-balancing system.

5) Guppies - One of my wife’s worries about the tank is mosquitoes. There is a little fish called a mosquito fish that lives almost exclusively on mosquito larvae. Guppies are a close relative and are readily available and inexpensive. They also consume mosquito larvae. Additionally, they are prolific breeders and stay small. There is a good chance that if I got a really healthy population going, they would serve as a good initial food source for the tilapia.

6) Catfish -. This is probably more of a hope than a reality. My real hope is that I can get a system in place such that I don’t have to buy food for the fish. I suspect that if I include catfish in the pond, I’ll probably have to buy food to keep them going. But I know that tilapia eat catfish waste and I know that catfish are bottom feeders, consuming organic matter on the bottom of the pond. So there is hope. Again, research.

7) Plecostomus - I figure one of these will be enough, especially if I can find a big one. He will definitely have to be big enough to not get eaten once the tilapia are introduced. But this guy, along with a little help from the snails, would have the job of keeping the algae down on the sides of the tank and keeping everything clean.

I am sure that there are plenty of other players needed. I will probably need something that will act as an intermediate step between the plankton and the guppies. The food web needs to be complete for the system to function as a whole. I think that even after the soil is complete, there will be enough nutrients washing through the soil to keep the system fed and active.

Once the tilapia are introduced, they should have enough food in the tank to begin reproducing rapidly and producing more fish. Ideally, the tank ecosystem will provide most to all of the food they need. As they get more numerous, I will supplement with black soldier flies, but that is another post.

As a first test of how well such systems might work, I introduced 8 guppies, 4 feeder goldfish, about a dozen tiny snails, and about 4 cone snails to a 55 gallon rain barrel I have on the side of the house. 2 of the feeder goldfish and 1 of the guppies died within the first 48 hours of being introduced to the system. However, the rest of them seem to be thriving despite the fact that I don't feed them. Actually, I do occasionally drop bugs in there to see what the guppies eat. They particularly liked the weed covered in aphids. But I definitely haven’t given them enough to eat that they would survive if they weren’t finding something in the water to eat. The guppies have even reproduced already. I know the tiny snails have as well, because they are already all over the place in there. So I will watch that system for a while and see what I can learn.