Brian Fanner wrote:
.....if you work on the fish to gravel ratio you need lots of gravel to provide lots of filtration for high stocking as per the norm of 3kg to 100liters of gravel. .......... In an ideal situation you would want as much as possible of both.
Big tank, masses of gravel filtration, clean water sump, pumping via a fine filter to nft returning to sump.
This ruling out more advanced forms of filtration like swirl filters or moving bead filters or drum filters due to cost and complexity and just the fact that such items are not really cheaply or readily available in Africa.....
Will it work? Yes. Will it achieve any real density of fish? No, excluding Tilapia, even then the growth will be negatively effected.
Recently, I have seen lots of talk about commercial systems using "hybrid" gravel and DWC/NFT system with an expectation of some unknown benefit. None of the benefits have been put forward yet. While the gravel will filter the water to a I doubt it afford you more fish growing capacity. I would have to ask how are you going to clean them?
Exchanging the water once per hour will still leave you with low culture density, more so if you are using gravel beds inline for filtration. Metabolically, the rate of oxygen consumption in the culture tank will beat you to it. Then the commercial viablility of the system will come into question. There is no reason why both aquaculture and hydroponics can not be commercially viable after all, these are your competitors.
The beds will certainly filter for a while, but eventually feed suspended solids back to the system. Those solids will reduce the growth of your fish and will have a negative effect on the entire system. What that impact will be depends on what form the solids are in (inorganic or organic). I know how much you guys don't like to talk about solids but in a commercial reality they are going to be an issue without adequate filtration.
Mineralization rates are very difficult to estimate because of the complexity of the systems that make it happen. Relying on the break down of your solids inline may be the undoing of the system entirely. Keep in mind that process will use both oxygen and will denitrify, either with or without oxygen. In other words a some of the nitrogen tied up in the solids and even the disolved nutrent you want for your plant growth, will be released into the atmosphere.
The infrastructure to manage the media beds will be quite large. Consider that the plumbing is used to transport the solid wastes to where you want them. This means to transport solids over long distances, the pipe velocities will need to be higher. To get higher velocities in the pipe work distributing the solids you will need either smaller pipes or much higher flow rates or even to the point of pumping them. Keep in mind you will need to distribute the solids evenly accross the gravel beds or they will clog very quickly at the entry point. Further to that, the entry point will become toxic and plant will not grow in it, losing precious space.
Then again after all that, at what level do you consider yourself commercial, supplying 3 or 4 resturants with fish?
Backyard Aquaponics
take a look at this man. He has an idea that makes a lot of sense when you really think about it. It is a new and different approach to aquaponics that sounds like it will be a lot better for the commercial future of ap systems. 
