Backyard Aquaponics
A place to discuss aquaponics

How many fish should I have with a 250g FT and just over 1000g DWC Floating raft bed? FT has a SLO pipe that leads to a swirl filter followed by a bio filter with oxygen supplied by venturi valve, and a 250g sump tank that I usually keep at about 100g or so. Right now I have a mix of 15-20 koi and goldfish and nothing will grow past seedling stage. Fish are between 3-6", just added 2 koi from my pond that are about 8". Don't want to over stock but nothing will grow I'm guessing due to the lack of poo. I'm feeding 2-3x/day with organic pellets. I've also been adding quite a bit of maxicrop plus iron but don't notice any difference.

0 Nitrites
0 Nitrates
0 Ammonia
7.5 pH

Stop adding the maxicrop plus iron for now unless you actually are seeing an iron deficiency. If you are seeing an iron deficiency then it's better to spray apply the maxicrop plus iron because at pH 7.5 in the system water, the iron would not be available.

Post up some pictures of you plants.

Your system doesn't have any filtration of fine suspended solids so there's a chance that the plants are dying because the roots are getting coated with suspended solids. How do the roots on your plants look?

Are you mineralizing the solids that you capture prior to the DWC? This can be as simple as aerating the solids in a bucket, you could even add some blood and bone meal to up the nutrients. Every day or two, turn the air off and let the solids settle, then add some of the liquid back into the system and turn the air back on in the mineralizer (add system water to fill).

Yes in OP i wrote that theres solids lift overflow into a swirl filter to remove solids followed by an aerated biofilter(has bio balls and netting)/mineralization tank
Just got my TDS meter today, at about 230 ppm total disolved solids

For the sake of background, there are several types of solids in a system

1. Settleable solids - these are the ones that an RFF or a Swirl filter can remove. Generally the longer the retention time the more that will settle out.

2. Suspended Solids - these pass through an Rff or Swirl Filter but would be caught by a Static Upflow Filter, a Drum Filter or some other type of sieve like filter.

3. Dissolved solids - This would be things like ions and minerals dissolved in water and these would pass through any of the filters mentioned previously.

The netting in your filter is a form of sieve filtration so it's a mechanical as well as a biological filter. With that in mind it should help with the suspended solids where a straight biological filter wouldn't.

and yes it's a mineralizer as well but it can remove nitrates from the system as easily as it adds them if you don't clean it often enough. What I was really looking for though is if you take the solids out of the RFF and mineralize them in a separate container or just dump them outside the system?

TDS/EC meters are very useful for hydroponics but not so much for AP where many of the nutrients are bound in organic compounds that probably are not detectable to the meter.

Your system setup is similar to what was used at UVI and they were recommending 60-100grams of feed per day for each square meter of raft. I'm guessing you have a bit less than 4 square meters of raft but I don't know the depth of your DWC so I can't be sure. This isn't the only measure of how much feed is needed but this is what worked for their system. Here are some feed rate tables - http://www.fao.org/docrep/s4314e/s4314e0s.htm

Here's a paper by Dr. Wilson Lennard that talks about system design parameters including various measures of the feed needed for plant growth - https://www.aquaponic.com.au/Fish%20to%20plant%20ratios.pdf