Highs and Lows of working with concrete

In the beginning of June, we decided to go forward with the construction of a concrete fish pond. I have little to 0 experience in construction and it was very frightening to rely solely on the Lord and his helpers here. There is an excellent construction crew at COTN whom I would recommend for any job, but it still is difficult to oversee something you have little knowledge about. After consulting various fish pond systems in Nigeria and in talking to the experts at the National Aquaculture Center in Malawi, we went forward and made two 6m x 6m concrete ponds side by side in three weeks costing around $3600 USD. We used
• 30MT riversand
• 25MT local 1.5 inch stones
• 75 bags cement
• 15000 local fired clay bricks
• 11 rolls 2ply brick force wire
• 12 15ft timbers
• 9 Y16 rebar
• 60m of linked fence
• 14 15mm x 15mm steel bars
• 3 63mm gate valves, 3 63mm PVC pipe and various fittings

To start, footers were poured on the edges with rebar inside. Brick walls were then erected at 2 bricks wide up to 1.5m height. The concrete floor was poured at 4 inches thick and smoothed and vibrated. The insides of all the brick walls were plastered and smoothed with about 1 inch of concrete. Two sides of the pond were completely supported with dirt backfilled although 2 sides were not. I did not think the walls had to be backfilled at all but unfortunately I was wrong and after filling one of the ponds overnight, we woke up to a broken side which we think originated at the weakest point, the corner. I was blessed to have a couple engineers present who were traveling on a flood team that were able to brainstorm solutions. I also found a very unique Malawian man who has constructed over 50 swimming pools throughout the country come and offer his advice. To fix the remaining pond which was largely intact, we spent another $550 USD to build a concrete pillar on the corner and fill in the inside corners with cement. Finally on July 11th and 12th we filled the pond up to 0.75m depth and began fertilization. Cassim helped do the first fertilization of the pond with chicken manure and was also taught how to take a pH reading of the water which is very important for a concrete pond as cement can be very alkaline. Cassim Tembo, a recent high school graduate has been recruited to work on the project. He has grown up in the COTN homes and is a very smart young man whom will help much. His dreams are to either become a doctor or go into business either of which he will benefit from learning the fish pond skills. There will also be a couple other students helping with the fish pond who will be participating in a business microfinance program which COTN is partnering with to help give their children the skills needed to transform Malawi and at the very least be self-sufficient.

Construction of the concrete fish pond

Concrete JM

Trial # 12: Damages of the concrete fish pond

Concrete JM2

Solution # 13 to problem/trial # 12:

Concrete JM3

Finished Product:

Concrete JM4Concrete JM5Concrete JM6

Concrete JM7

J. Minich

2 thoughts on “Highs and Lows of working with concrete

  1. Hello,What are you going to stock your pond/s with (species)? Are you going to use mono-sex fish (all-male)? What are you going to feed them with?Stocking rate? Filtration? Have you worked out the perilous logistics of fertilizing small ponds? It seems as though a lot of forward planning needs to be done. to make this expensive project a success not just another African failure….


    Nick James
    Rivendell Hatchery, Grahamstown, RSA
    Tilapia aquaculture consultant.

    email nickjames@intekom.co.za


    1. Hi Mr. James,
      These are great questions indeed and I hope that this will not turn into an all too familiar African failure. As you may be aware, Malawi does not have endemic O. niloticus, but rears O. shiranus (omnivore), O. karongae, and T. rendali (herbivore) as tilapias are concerned. For our initial harvest we have stocked 60 O. shiranus and 60 T. rendali (mixed sex). Obtaining sex reversed tilapia here is a challenge as I believe the government may still have laws against it. In the coming weeks, we will add 150 Clarias gariepinus- African catfish bringing the stocking density to 7.5 fish/m2. In the future we will aim to stock around 400-500 C. gariepinus and maintain some tilapia broodstock for occaisional fingerling production to sell or feed to our catfish.

      For water quality, I have an aerator that is being installed this week that will generate 65 gph of air and uses a mere 12 Watt of power. We will replace 25-50% of pond water weekly using the water for irrigating about 500 tomatoes, maize, and other vegetables (rape seed, mustard, and likely some fruit trees). Did you have another concern with regards to filtration?

      As for working out the fertilization regime, I am currently doing some experiments at Malawi’s National Aquaculture Center in Zomba to look at various animal fertilizers on fish production (O. shiranus and C. gariepinus) in concrete ponds. I have found very little literature on others fertilizing concrete ponds, but it is my instinct that the microbes involved will have a better effect than not fertilizing at all. In any rate, we are being extremely careful at first to ensure the water doesn’t get too green. We started off fertilizing at 500Kg/HA/week which is 2Kg per 40m2 and got green water in 2 weeks. We have dialed it back now to 250Kg/HA/week and are ensuring to drain and fill on a weekly basis. Our water source is from a bore hole.


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