Backyard Aquaponics
A place to discuss aquaponics

I have a 200 gallon FT, and a 265 gallon sump, run the sump around 150-175 gallons. Currently 6 goldfish, and if I remember right 31, 32 tilapia. Can I add more fish? GB's are 336 gallons and I also have 6- 5' towers and 1 DWC bed appx 80 gallons. I have lots of tilapia waiting as they are now breeding in aquariums. Also, thoughts on adding a different species of fish.

length x width x depth of growbeds (filtration)
Then divide by 20

1 standard 350mm high IBC growbed (1000x1000x350 / 20 ) = roughly 17 fish per growbed of this size
to err on the side of caution, probably /25 would be better. (14 fish)

It also depends on the media being used,crushed granite or lava rock has a much higher BSA than say river gravel. You can’t count your DWC in your stocking and the towers only if you have media that supports bacteria,as for the sump its best not to count this also as this is a reservoir of clean water that you are pumping into your fish tanks,the clean water returning from your grow beds,so with 200 gallons stocking conservatively at 10lb per gallon you can have 20 fish.

This is not really a consideration unless you are removing solids. In a system where GBs are responsible for filtration of solids there is masses more surface area for biofiltration than you need. The GBs are as big as they are so that there is enough space to store the solids while they are processed. Therefore it is the space between the particles that is important not the surface area of the particles.

System water volume shouldn't be used to determine safe fish stocking capacity, at least not as the primary determining factor... as VK mentioned, wet gravel should.

Once you've determined how much fish your system can safely stock in ratio to gravel, then a gravel:water ratio of anywhere between 0.6:1 and 2:1 will be fine, many people use 1:1 as standard.

ie: A system with 1200L of wet gravel and 2000L of water (total system water), can safely stock 48 Trout or Barramundi (require 25L+ of wet gravel per fish)... at a 0.6:1 gravel:water ratio

A system with 1200L of wet gravel and 1200L of water (total system water), can still safely stock 48 Trout or Barramundi... at a 1:1 gravel:water ratio

A system with 1200L of wet gravel and 600L of water (total system water), can still safely stock 48 Trout or Barramundi... so a 2:1 gravel:water ratio... but in this last scenario your back-up air system would definitely need to be adequate and very reliable.

Your previous comments on ratios of 1:1 vs 2:1 vs 1: 2 make more sense now.

Couldn't agree more wish I had understood earlier.

I don't really like quoting those ratios, because many people are confused by them... probably my delivery... and I don't really agree with the 2:1 gravel:water ratio, IMO it's too extreme in regards to the number of fish for the total water volume, but it's actually recommended by a number of people, one in particular with a lot of letters after his name.

Those that do recommend the 2:1 ratio tend to have come from aquaculture background, are focussing more on commercial scale systems, and I guess don't mind packing the fish in close quarters. But for your average backyard system I tell my customers to aim for a gravel:water ratio somewhere between 0.6:1 and 1:1... most single IBC systems for example are running around 0.6:1 and getting good results.

I do agree with you but if you had two pieces of media the same size,one with a smooth finish the other with a rough irregular surface wouldn’t the later be be efficient in terms of surface area.

Okay maybe I have the complete wrong idea about GBs acting as the biofilters or I am misunderstanding you... but I thought the most important aspect is the total surface are for bacteria to grow on not the volume of space that it occupies.. isn't that why media made for biofilters in aqua culture and sewage treatment are up in the 400-600+ SSA compared to gravel or expanded clay which is perhaps 45-60 SSA? and that is why GBs must be sized larger than other types of biofilters with specially designed media like K1 etc?

Although in GBs I would think that sometimes the higher surface area of something like scoria wouldnt matter as much in the long term as the small porous holes in that media would get blocked and unless cleaned off wouldn't offer the benefit that would appear theoretically.

To clarify, I'm only removing solids going to my DWC raft bed, which is 3' x 8' x appx 5" depth. I have a total of 3 GB's, with them being 3 different media, 1 volcanic rock, 1 cedar bark stone( a hard stone with a relatively smooth surface that is sized 3/4- 2" in size), and 1/2- 3/4 " river gravel. Total of all beds is appx 300 gallons(appx 44 cu ft) of material, not including the 6 towers which all have media of some type. Mostly a poly filter material. My nitrates are consistently at zero, so was thinking a few more fish may be in order.

GB's in aquaponics are more like an all round biofilter. Yes the media provides surface area for colonisation of bacteria and microbes etc but they also provide voids for aeration, digestion and mineralisation of wastes. Like Stuart pointed out, the surface area in a standard media bed provides more than enough surface area for colonisation. In aquaculture, an MBB/ biofilter using something like K1 is basically for colonisation only, hence the large surface area materials used. Apples and oranges.

How many 336 gallon GB's?... and if that's their quoted capacity full to the brim, what's the realistic wet gravel total of each one.

None of us Aussies have experience with Tilapia, but I from what I've read I'd suggest 20L+ of wet gravel for each plate sized fish. You'll be able to stock heavier when they're younger and progressively thin out the bigger ones if/when you start getting high Ammonia or high Nitrate readings.

Edit: I just read your post about your 300 gallons, so that's 1135L of media, if that's mostly wet media then you should be able to have 50-55 Tilapia, especially if you stagger their sizes, and like I said before, if/when you start getting Amm readings, or excessive Nitrates, just cull a few of the bigger ones.

Good point Damage. My total is 45 cu ft, or 336 gallons of GB, but they are not full to brim, and then the siphon don't allow for 100% of the area to stay wet. So, now I'm down to closer to 300 gallons or 1135 liters. From your guide, about 56.75 fish. Not sure which one gets that .25 removed...draw straws maybe. Thanks, I think I can add a few more without any real worries. So far zero ammonia, nitrates, and nitrites since cycling. PH remains on the high side 7.4-7.6. I keep topping off with my hard water, and haven't been as good at regulating the PH with acid prior to adding like I was earlier.