There seems to be some ‘confusion’ about feed rates and I believe we’ve mentioned this far more than once in some blog posts and/or comments (both feed inputs and composition definitely covered in publications).  Rather than trying to find that or asking you to do so, I’m copying below some more recent responses to feed related questions.  
Do note that all this is strictly in regard to Tilapia with a balanced commercial feed (in my case of a known elemental composition).  I highly encourage others who are serious about plant production to have their fish feed analyzed (by a qualified lab) for the plant essential elements (yes, it will have a cost, what doesn’t? – except my efforts and time that is ! – to date).
 If you have the option, feeds ‘fortified’ with mineral and vitamin supplement ‘packs’ are not necessary.  IMO, knowing what the amount of plant nutrients are in a particular feed will be useful information for you to have (and report).  If you have your’s tested, report the results to us and I/we will provide feedback.
 
Note: Anyone growing goldfish or other carps, perch, bluegill etc. should also know (IMO) what their feed composition is and I can offer ZERO suggestions as to what the appropriate feed rates (or temps, DO, etc) are at any size or density for any species except tilapia.  Also, if you have some prior experience with gravel or clay-pebble media, you will find being able to feed at a much greater rate when using sand as the filter media.  (and BTW, the word IS “media” and not ‘medium’)
 
=========
 
RE: report of ‘slow’ plant development with indications of nutrient stress
 
If I’m understanding you correctly:   ‘wastes’ from 80 grams/day distributed over 30 m2 = 2.67 g/m2/day … which is ‘basically nothing’ – aka, no where near enough nutrient to support vigorous plant development
In the ratio studies (all-male Tilapia):
Feed given over the first 31 days starting with N= 80/m3, 15 g fingerlings  (includes 1 week at very low rate at start-up before ‘cycled’) = 3.43 kg/m3 (per tank, 31 days)
At v:v 1:2,  aka v:a 1:6   =  572 g/m2 (filter) or 18.44 g/m2/day (first 31 day average)
I also looked up the feed input rate data over the first 103 days.
Summary:
  • First week input rate at 4.5% of biomass/day tapering off to 2.1%/day (last week avg.)
  • Total feed (mean) per individual = 257 g
  • Input rate as % of Pm (day 7)= 9.7%/day
  • Input rate as % of Pmf (day 103) = 1.8%/day   
    • BTW:  “P” = individual, “m” = mass (weight), “i” – initial, “f” = final
The one-year feed input rate averaged 133 g/m3/day …. Which at v:a 1:6 = 22.17 g/m2/d. 
  • 2/3rds of that time was with fish larger than 300g (reduced feed as % of biomass/day and w/ lower FCR than smaller tilapia).  
    • BTW: feed rate %/size and FCR are NOT linear functions
  • also included a 40+ day interval without any plants (intentional), then another month trying to raise pH again (with a reduced feed rate)
SO … I ‘feel’ that approx. 150 g/m3/day long-term average is a viable target (perhaps more w/ continuous/vigorous fruiting plant production).
At v:v 1:2, then v:a= 1:6 , so feed rate (‘wastes’ from) per m2 of filter in the 25 to 30 g/day range as a long term average (until determined either excessive/insufficient).
SO, if you can’t get the stocked fish to consume enough feed to sufficiently fertilize the entire biofilter area you have available, then temporarily reduce the portion of filter being irrigated/fertilized until they will eat enough to support more plant growth.  This can be accomplished by temporarily blocking off the furrows (length of) to limit the area receiving nutrient and just grow plants (initially) in that section until you are able to feed at a higher rate and increase active filter/plant area.
 
==========
 RE: Another response to similar question 
My one-year feed input rate averaged 133 g/m3/day (all-male tilapia)
2/3rds of that time was with fish larger than ~250g (reduced % of biomass/day and w/ lower FCR with increasing size)
also included 40+ days without any plants, then another month trying to raise pH again (reduced feed rate)
SO … I ‘feel’ that approx. 150 g/m3/day long-term average is a viable target (perhaps more w/ continuous/vigorous plant production)
At v:v 1:2, then v:a= 1:6 , so feed rate/m2 of filter in the 25 to 30 g/day range (until determined either excessive/insufficient). During a system start-up )with small fish and young plants) one can feed as much as the fish will eat twice a day and the plants should be okay (was for me) with the feed input increasing as both the fish and plants grow.  I believe that my initial feed rate during the first start-up ( still ‘cycling’, microbial populations just starting to develop) was 40 g/m3/day (at v:v 1:2 = 6.7 g/m2/day and was at 120 g/m3/d (20 g/m2/d) or more within a couple weeks.
 
==========
 
What is the optimum water:sand ration? 1:2.4 or 1:1.5?
Depends on several factors, but over the long-run, the amount of feed input determines the amount of fish ‘waste’ generated, and the amount of ‘waste’ being accommodated (on average, over time, many months to a year) would dictate the appropriate  filter volume.  More feed = more ‘waste’ = more filter surface and more soil biology to process.
We have been suggesting v:v =1:2 (or v:a 1:6) for first time adopters as suitable for from 80 to 100 tilapia  per cubic meter grown from 15 g to 250+/- gram in 3 to 4 months assuming they are feed a balanced ration fed all they will eat twice a day. Once they’ve attained 250 g (approximately) then the ‘standing biomass’ should be reduced (by culling or increasing volume(s)).
Since I’m mainly interested in the ‘wastes’ (more than the fish production), one could also have greater fish numbers and feed them less intensively (grow more slowly) or even fewer fish and fed very aggressively assuming that water quality remains ‘good’ (allows)
What is the fish density? by weight or by number? assuming 15 grams fingerlings?
In the ratio study. I had N= 80 m-3 at Pmi =15 g  – they did ‘fine’ across all the v:v ratios from 1:0.67 to 1:2.25
At v:v 1:2, one should be able to do well at N=100 m-3 (reduce N when attaining 250 +/- 50 g  average size).
Growth rate and FCR was slightly better in the higher ratios due to ‘cleaner’ water. Plant yield per plant and per area was better at the lower ratios but total plant yield per filter (not area) and per gram of feed input and per fish growth/increase was greater at the higher ratios.  The combined yields per unit feed input was found to be between the 1:1.5 and 1:2.25 ratios (under those prevailing conditions), So, I’ve suggested 1:2 for beginners for both simplicity and leaning toward the larger biofilter capacity to error on the side of caution (for the fish).  Also note that this was with (for) indeterminate tomato and cucumber and not for leaf crops, legumes and/or cole (Brassica) family.  
==============
 FURTHER COMMENTS:
There is no magic one-size-fits-all number for feeding rate.  All feeds vary in composition, All fish species and growth phases have different requirements (upper and lower limits and an unknowable optimal rate on any given day), All sand, water qualities and locations will be at least somewhat different – overall and as time/seasons progress.  All plant species and phases of their growth will have somewhat different nutrient uptake rates (by amount and element – each and every day).
The fish will indicate through their feeding behavior if they are hungry (when full).  The plants will ‘tell you’ by their growth rate and any foliar symptoms that might develop if they are getting enough nutrient or not.
As a ‘system’ matures (elements accumulate), feed (nutrient) input each and every day is not critical from the plant’s ‘perspective’..  Longer term (monthly) feed rate, aka ‘waste’ generation should be approximately balanced with the prevailing average plant nutrient uptake.  As the ‘soil’ develops, as with an in-ground garden, there is no need to add fertilizers to the media (plants) each and every day (or even week).  Sure, the fish will want to be fed every day – but provide enough feed ‘run through’ enough fish to supply the nutrient demands of the plants (number, species) you are growing.  Not more (long term), not less (long term).  Balance.
There are dozens of variables ‘at play’- every day,  changing one aspect/parameter WILL influence everything else eventually,  This is the ‘nature’ of an (every) integrated “SYSTEM”. Know what is actual happening in your system (pH, temp, DO, feed input/quality, FCR, etc.) , Observe what is happening carefully and often.  Observe, assess, adjust, observe, assess adjust, repeat, repeat repeatedly.
REMINDER”  Plant nutrition is STRONGLY influenced by pH. … with 6.4 to 6.5 generally ideal for most species and elements.  I suggest the outer limits for good plant nutrition/growth to be pH 5.8 on low (acid) side and 6.8 on the high side.  Most AP seem to claim/prefer to exceed pH 7.0.  This is NOT ‘good’ (recommended).  Facts are facts and unsubstantiated opinions aren’t.  Believe it or not.  Totally your choice.  Every and ALL content I/we provide is exclusively intended to be to your direct benefit.  Argument (absent evidence) is not appreciated.
And, as another reminder, iAVs is 90+% about plants.  Fish growth is a means to an end.  It’s the metabolic ‘waste’ products (and microbial ‘processing’ thereof) from the fish growth (feed) where the value  resides (both economic and nutritional).  If you are merely interested in fish production and with tinkering with equipment, plumbing and sundry gadgets&gizmos then (IMO) stick to standard recirculatory aquaculture practices (not so-called ‘aquaponics’ – whatever it is you ‘think’ that is/involves)  ! Seriously !

Pin It on Pinterest