Backyard Aquaponics
A place to discuss aquaponics

Was reading an Arizona State AP study that says optimal feeding rates are 60-100g per 11.5 sq ft growing area. I have 60 sq ft , not including a few vertical towers. High rate would be 19.4 ounces per day! That would be 6.5 ounces three tines per day. Not even sure if my 40 tilapia could eat that much. I'm currently feeding then about 5 ounces per day. Is it possible, or advisable to try and increase by 300%. I could also go at the lower rate, but since I have lots of fruiting plants, thought the upper end was closer to match my system needs. Can 40 tilapia, average size 8" consume almost 1/2 ounce per day? I have zero Nitrates, but growth is decent. PH is 6.6, ammonia at .25.

I've found that my fish eat as much as they eat!

You're starting at the wrong end of the computation: you've got 'X' fish which weigh 'Y', you feed them 'Z', now figure out much growing space that equates to.

That was my other question, do I reduce my grow area? Also, how much can I feed my fish...as much as they will eat in 5-10 minutes?

I had 2x 400mm high growbeds - calculated 84g of feed per day, which is around 2 large handfuls

I can't find the equation anywhere - I want to show my students. It was slightly different for different media i.e. scoria vs pebbles. Anyone?

Ive found my fish eat varying amounts depending on a lot of factors, some of which I havent isolated.

So, i follow 3 rules so that i can overfeed.

1. Only feed when i can observe (daylight, when i have time to keep an eye)
2. Start with a small amount, keep feeding until they stop eating
3. Fish out any uneaten food with a net.

In the summer, I have fed the fish up to 3 times a day.

Are you sure this is an accurate quote, Chris? Feeding rates have much more to do with the volume of the grow media rather than the growing area or surface area of the grow bed.

If you work out the total volume of your media then you can calculate the surface area of your media (as distinct from the surface area of your grow beds) based on the media type (there are tables covering this). If you add this figure to the calculated surface area of all other submerged components of your system, ie grow bed and fish tank walls and bottom, interior surface of pipes and so on, then you have a total figure for the area that can be populated by nitrifying bacteria. It is this surface area, rather than the growing area, that determines the capacity of your system to break down and convert fish feed and/or fish waste. As is always the case, though, there are also other factors to be taken into account.

There are numbers somewhere on this forum and in the literature which can be used to determine maximum, or at least safe, feeding rates per unit (sqm or sgft) of bacteria colonising surface area. We refer to this as the 'filtration' capacity, which is somewhat of a misnomer. If the figure you calculate for your system is, for example, 415g of feed per day then I'm fairly sure that means you could 'feed' your system that amount of feed and the bacteria will process it, regardless of whether it has been eaten by a fish and converted to waste or if it remained uneaten. In practice it would make sense in your case to feed your fish no more than they can eat at a sitting if only to avoid waste. It probably also makes sense to back off a bit from the calculated figure so that you have a safety buffer.

It seems that you may have sufficient filtration for more fish than are in your system, which is a good place to be. My own system, in contrast, has insufficient filtration for me to feed my fish anywhere near the amount that they are capable of eating. They are always hungry. If I was to use the rule of thumb that one feeds ones fish as much as they can eat in 5-10 minutes then they I would have hundreds of dead Rainbow Trout inside 24 hours from nitrite poisoning.

I don't think you should, or need to reduce the size of your GBs. Some systems on here, especially one at the BYAP/Cheidys Aquaponics Shop run really well with a low stocking density.
The "optimum" figures that you found could well be more focussed on maximising fish production.
Having extra beds would only give you more stability and room for error. (Within reason of course)

They were talking veggie production.

Then my next solution to gain more nutrients would be add more fish in my current FT...or add another FT? r dose with more Seasol/Rootmagic(a like product that I can get wholesale). I was able to feed about 50% more yesterday, but this AM, they slowed down a bit and I had to skim off some food. Also my Ammonia went up from .25 to .5. Not too worried at this point, but will monitor closely and if it continues to rise, will back off feed rate.

It's quite hard to under-feed fish; it's very easy to over-feed fish (some species will eat until they die!)

How do you know your plants aren't growing at the optimum rate already? How old is the system?

Lot's of tests, experiments and collective experience has shown that great crops can be grown with minimal numbers of fish being fed a subsistence diet...

Bunson.good point. I'm sure a lot more fish are killed to to over feeding than under feeding. System is just over 1 year old. I'm going what research I have found regarding nutrient levels and AP systems. Most seem to agree that a Nitrate rate of 60 -100 for lowering plants. I think Ryan Chatterson may even run higher rates for his amazing results. As i would assume most people want, I want to try and produce the best produce possible. My research shows that in order to do that, I will need higher nutrient levels. Now, the problem is: How do I get them without adding lots of additives(besides Iron, Calcium, and Potassium)? My answer is add more feed. Now, have to monitor and make sure that I don't cause problems with adding too much. Also working on MT that will hopefully help by recycling some of my filtered solids. Growth is not poor, but nowhere near where I would like to be.

Just FYI 60-100g/m^2 is Dr Rakocy's UVI feeding rates.

I would feed more. The problem is... Can your filtration handle the increased feed amount? It's hard to design backwards after the fact but hopefully your current filtration can handle it.