Backyard Aquaponics
A place to discuss aquaponics

I have a basic IBC tote system set up in my backyard greenhouse. The pump is in the 150 gallon fish tank where the water is pumped to the growbed filled with hydroton media. I have lots of plants in there growing well. The tank is stocked with blue tilapia. The system has been running very successfully for 6 months.

In the past few weeks, the ammonia levels have climbed to alarming levels, although the fish are showing no signs of stress. Nitrites are zero, nitrates are moderate to high. PH is 7.6.

After doing a partial water change a few weeks ago when I noticed it, I have since been keeping the levels down by hardly every feeding the fish. This is not a long term solution. If I feed them even small amounts, the ammonia levels quickly start to rise.

Here are some things that have been suggested as my problem:

1. Overstocking. I have used the charts, my own common sensibilities, and have sough the advice of local professional aquaponics experts and it would seem that I am not overstocked.
2. My bacteria somehow have experienced a die off. If so, why? And why then would the nitrites be zero but nitrates still consistently high. remember my system has been fully cycled for months. I have a few red worms in the media that are still alive after all this time, so wouldn't they be killed if something like high temperatures hurt the bacteria?
3. Overfeeding. I understand this is a common problem, but I assure you I am way underfeeding, even before the ammonia increase was noticed. Now I am hardly feeding at all just to keep it within safe levels.

The PH is also oddly high. I know the natural processes drive the system PH down, which was exactly the case up until I noticed the ammonia issue a few weeks ago.



I have consulted experts and read everything I possibly could online about this issue, and so far I have come up empty. Any input would be much appreciated!

How many fish do you have in there?

You don't have a dead fish stuck in there somewhere? Like under the bottom of the pump?

As they are so fast, it's hard to count them, but I think about 40 tilapia. The biggest being probably 7 inches/18 centimeters with the smallest 2-3 inches/6-8 centimeters. Most are on the smaller side.
I have also installed a zipgrow tower with media matrix to serve as additional filtration.

No, I have been looking for something like that and have found nothing.

I had trouble finding them too. I had to turn off the actual pump, and pull it out before I found it. Was a sneaky little sod.

I'll also point you to Coachchris's thread where he had a similar problem:
viewtopic.php?f=8&t=22648&p=470124#p470124

I will keep looking. Thanks for referring me to the thread. My old pump stock working about a month ago, so I've got a brand new one, certainly not clogged up yet, but I will keep looking. Very frustrating issue.

Is it gravity fed to the GB's? Sometimes the gravity fed pipes clog up with solids.

I wouldn't take any further advice from those "experts"... you are simply waaay overstocked.

An IBC GB with 30cm(12") gravel depth will be holding around 320-330L of gravel, and when flooded to about 40mm(1.5") from the surface will provide around 270L of WET gravel.

I can't quote exact figures for Tilapia, as we can't have them here, but as an example, fast growing fish such as Trout and Barramundi require 25L+ of wet gravel per fish to get them safely to plate size of around 600gm cleaned... so 10x fish per IBC system... and I would imagine Tilapia would have around the same bio-filtration requirement as they are also fast growers. If you try and push the envelope, even by just 10%, you will have Ammonia issues.

Even Silver Perch, which are a much slower growing fish, require around 20L+ of wet gravel per fish, so a safe capacity of only 14 in an IBC system with 30cm(12") gravel depth.

I have been told that my stocking density is with my tank having 10-15 full 1 pound Tilapia. Does that not sound correct? I have around 40, but most are still quite small, I don't think the total weight would add up to be that much currently.

lol I got close to 100 talapia in a 750L pond.. you can't be more overstock than I am. I usually don't get amm spikes.. my spikes are usually nitrite. quick questions.. 1 did you change you feed brand recently..? have you increased your feed amount recently. are suplementing and what are you using. you average system temperature. and are there any clog spots in your GB. I'll leave you with these questions for now.

one major question is do you have any sort of mechanical filtration. and when was the last time you cleaned it..?

My temperature fluctuates between 79 and 83 F, 26-28 C

Did not change my feed brand, and hardly feed at all. I can get the ammonia to zero, but when I feed even a very small amount, it goes up and and I don't feed again for a day or two. The fish are certainly underfed currently, have been for nearly 2 weeks.

Clog spots in the grow bed -- it could be something like that. There are no bad smells anywhere and the plants look amazing, what would I look for and what would I do about it if there were clog spots?

The hydroton growbed and the zipgrow media based tower are the mechanical filtration. I also put a kitchen strainer as well over the input to the growbed and change it out daily, although there are very little solids in there, just enough to clog the tiny strainer holes.

Otherwise, anybody got any good ideas on how to kill, process, and enjoy small tilapia?

Thanks!

10 x 1lb Tilapia would be fine in a system with an IBC GB with 300mm gravel depth and flooded to within 1"-2" from the surface of the gravel... but 15 would be a stretch IMO.

Final harvest weight is a reasonable gauge of how many plate sized fish your system can safely stock, but especially when your fish are young and rapidly growing, your safe fish stocking capacity has more to do with the amount of feed that's going through your system, not the total weight of all the fish in the sytem. A young rapidly growing fish that may weigh only half of the final desired harvest weight can still eat as much as much as a harvestable fish... Think about it from a human perspective, who would eat more... 40 x early teens, or 10-15 adults?

Feeding at a low rate is fine, but you have to take into account that fish produce Ammonia even when they aren’t feeding.

Also, IMO, you should make it habit to refer to fish stocking capcity in ratio to wet gravel in your system, not your FT volume.

I’m surprised you would make such a statement Cookie... relating fish stocking capacity to water volume, especially without referring to, or asking about the O.P’s bio-filtration capacity.

Talk to your neighbours, show them your system, help them build their systems and then sell them some of your fish... but work quickly!