As we have said before, there is plenty of information around the internet on aquaculture and aquaponics, some of it great and some of it interesting. One of the confusing topics we see discussed is around media bed sizing for backyard aquaponic systems and from those discussions have come a very wide, vague and confusing results. I am certain we will clarify some of those for you here. The two requirements in aquaponics is filtration for the fish and nutrient for the plants which can cause confusion, especially when the two are in one place; the media bed. The media requirements for nitrification are, just like any other bio-filter, linked to the fish feed. Most times the nutrient requirements of the plants will make the media beds larger than required for nitrification, but we find working both the plants and the fish out separately and pick the largest is the best starting point. Let’s say you would like to grow 1kg of fish, which is more like a large aquarium than a food production unit, but it makes the math simpler for us all to work in small numbers. So, if you were to have a standing stock of 1 kg of your favorite fish, based on nitrification or TAN (Total Ammonia Nitrogen) removal needs alone you would need 10 liters of wet media per kg of fish. Simple enough? Most home aquaponic media based systems will generally have more than 10 liters of media in their grow beds, so let’s work from the fish culture water volume point of view and expect most can grow 25 kg of fish in 1000 liters of water or as density is commonly referred to as 25 kg/m3. If you know the volume of the tank water you want to grow fish in your aquaponics system, we can work on a simple rule of thumb 250 liters of media per 1000 liters of fish tank water for nitrification or TAN removal. Remember the result is based on 25 kg of fish in that 1000 liters being fed enough to grow them sustainably. Now we can take a look at this in another direction and work from the feed input, which is my preference. Generally though you may want to know how much to feed your fish per day. In general terms for basic design considerations, you can use 1% of body weight per day (1%bw/d). This means the weight of all the fish in the tank at its maximum capacity x 1%, which gives you the feed per day. Sticking with the simple math and feeding 1kg per day, this would need 1000 liters of grow bed media for nitrification biofiltration. To put a bender on that for you it would equate to 100 kg of fish per 1000 liters of wet media. Remember the first calculation we did per kg of fish = 10 liters of media? Now we have 100 times the weight in fish so it all connects no matter which way you want to work it out. You will see the same result for the liters of water only we would need 4 times the water to support 100 kg of fish. For example 4000 liters to grow 100kg of fish at 25kg per 1000 liters. Making sense? To apply a bit of known science and add some safety margin in for good measure you can easily use the following very simple “rule of thumb” for working out how much media bed you will need. We have based the table below on a grow bed media with a surface area of 100 m2/m3 To sum it up (Media Surface Area 100m2/m3); Fish Weight 10 liters of wet media per 1kg of fish Water Volume 250 liters of wet media per 1000 liters of fish water Feed Input 1000 liters of wet media per 1 kg of fish feed per day How to use the table: Looks too easy really. If you have media that has a surface area of 300 m2 then divide any of the media volume results by 3. If you have media which has a surface area of 800, then divide the results by 8. You get the idea. Don’t worry too much about protein levels changing, there is plenty of “fat” in the above equation which we will explain in more detail later. An important point to note is “Wet” media. Most media beds are flood and drain or continuous flow so have different volumes of media in contact with the water depending on the users preference. Flood and drain beds tend not to flood the top 50 mm to 100 mm of the bed depending on the media used to reduce the sun exposure on the water and constant flow generally will not use the top 100mm depending on media type and rely on the capillary action of the media. So it makes sense to only count the areas the bacteria will use (the wet media) when working out bio-filtration capacity. Some of the assumption in the above results are 45% protein feed and 100m2/m3 surface area of the media used and 25 kg/m3 fed at 1% per day. You MUST have adequate pre-filtration for solid fish and feed wastes before the growbed or biofilter. If we wanted to really simplify that it would look like this : and How much feed example 1: You have media like gravel that has 300m2/m3 surface area You have 84 liters of it. From the formula above 300 / 100 x 84 = 250 grams of feed per day How much feed example 2: You are using kaldnes plastic media that has 850m2/m3 surface area You have 30 litres of it. From the formula above 850 / 100 x 30 = 255 grams of feed per day How much media In words = 100 divided by the surface area of the media you have multiplied by the grams of feed you feed per day. How much media example 1: You have a media like gravel that has 300m2/m3 surface area You are feeding 250 grams of feed per day. From the formula above 100 / 300 x 250 = 84 liters of media required for the biofilter. How much media example 2: You are using Kaldnes plastic media that has 850m2/m3 surface area You are feeding 250 grams of feed per day From the formula above 100 / 850 x 250 = 30 liters of media required for the biofilter. Remember this is for nitrification or bio filtration only…. Not a fish to plant ratio. Understanding this is for nitrification only and takes no consideration of solids accumulation in the media, which we have discussed previously here. We will account for the solids accumulation and degradation of nitrification in a media bed in a later article….