Making tanks onsite is my preferred method when building farms because shipping costs for “air” is expensive especially if the manufacturer is not close by. If you have some skills to put tanks together onsite or you can contract someone to do it for you, a costing exercise is certainly worth it. What you will find with making tanks onsite, is once you get down to around the 1000 liter size it is sometimes cheaper to buy the premade tanks. This means that your Quaternary may be less expensive to simply buy the small tanks (500 to 1000 liters) and plug them in compared to fabricating small tanks onsite. One thing you will have to decide about your DIY culture tanks is the depth of them keeping in mind you will have around 100 mm of free board so the tank is not filled to the brim. Note we have chosen a depth of tank at 1.4 meters. This is allowing for the volume we require, then you add your free board to the top of that. In our scenario above, we are looking for a 1.5 meter tall tank filled to 1.4 meters. A few things to note about the above chart. The diameter of the quarantine tanks is far too small and we know this from the diameter to depth “Ratio Check”. The ratio check lets us know if the tank will spin too hard (tall and narrow) or spin not fast enough (wide and shallow). Aiming for a diameter to depth ratio close to 3:1 but not exceeding 4:1 is a good place to be. Decreasing the depth of the tanks will naturally increase the diameter and diameter to depth ratio. As you can see above we have changed our depth to 1.1 meters water depth (1.2 meters tall) and it has given us a reasonable configuration of the tanks we want to put in the fish shed. You may be limited by space and want to have taller tanks. Be careful going too tall and keep an eye on the ratio. I prefer to have all of the culture tanks the same height, but the quarantine, because it is a separate system can be any height that works. As we noted earlier you may want to purchase premade small tanks rather than make them onsite. Tank Ratio Sidebar: A client came to me years ago with a plan to build an aquaculture farm. He was set that he would use sheets of iron used for making large water tank rings and line them. There is absolutely no issue doing this. However, these proposed tanks were 11 meters wide and 1.1 meters deep. This gave him a diameter to depth ratio of 10:1. It was 2.5 times higher than recommended which did not appear to give him concern. When I explained it would be impossible to generate the velocity in the tank to move the solids from the outer part of the tank to the center outlet. He explained to me he would put some type of sweeper in it. I advise do not put anything in your tank that does not absolutely need to be there. I have enough troubles with pipework in a fish tank. Then I described what I had seen similar tanks do. What happens in the inner and outer velocities in the tank become at “odds” with each other and they start to create waves in the tank. Not tsunami waves, but a wobble on the surface of the water which spills water out of the tank everywhere once the tank is turning at speed. Not only can you not move the solids out of the tank quickly or at all, they make for messy fish tanks dropping fish out of them as they “wobble”. This is why there are preferred diameter to depth ratios for aquaculture tanks. Something to keep in mind. Building Tanks Onsite While this series is focused on the design criteria rather than the equipment, I thought you would find it useful to see what it looks like building tanks onsite from a few of our projects. These are large tanks so don’t let them turn you off approaching your build in either one of these ways….
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