Aquaponic Gardening

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Hello Friends! Over the weekend, I was invited by a nature farm to have a look at their massive pond. They plan to incorporate aquaponics in their current set-up. I noticed they used a pond liner to hold the water. According to the farm manager they started to fill the pond with stream water on August of this year and the following month they stocked about 500 size 12 Tilapia fingerlings. Barely 4 weeks has passed all of the fish died. They drained the water and removed all organic materials from the bottom of the pond and filled it half-full this time. The pictures below shows how it looks now after two weeks of reintroduction of 500 fingerlings. 

I tested the water using API freshwater test kit ( pH - 7.6, Ammonia- 0.25 ppm, nitrite and nitrate - 0.0 ppm). Air pumps are running 24/7 and circulating water pump operates the water falls during day time. No separate solids filtration system, No separate biological filter system. So I asked them to test the water again same time next week for increase in nitrite and nitrate. I was wondering, with only 500 fingerlings what could possibly caused the fish kill? Any suggestion, idea, advice on how to incorporate this pond with aquaponics? What would your approach be? will multiple small scale system be better than a massive solids filtration and biological system? I want to know everyone's take on this.

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Maybe a quick temperature change or sudden drop in oxygen?
Does this pond have bottom drains?
I had to design a system recently for a similar size pond. I'm assuming they want to eventually stock as many fish as possible, with such a large pond you'll want to build as many grow beds as possible to filter the water. I specified sieve filters to efficiently handle solids at the high flow rates needed to keep a big pond clear. If you can't get or afford them, then you can custom build some settling tanks.
You really don't want solids to build up anywhere in a system this large as it will be a really big pain to clean out eventually.
The grow beds, whether rafts or media, will provide all the biological filtration you need.
If they don't want to take up much space growing tons of food, then you'll be forced to create a specialized super efficient biofilter just to keep the fish alive.
It all depends on their priorities and how much space you have to work with.

Hi Averan, They definitely need to set priorities. Yeah they do have bottom drains at the center which is the deepest part of the pond. The first time they drained the pond, they were able to scrape out almost a foot thick mud/waste, I saw and observed the sun dried waste, it appears to be cement-like that might have been washed away from the surrounding structures and settled at the bottom. I assume it to be the culprit, It must have caused the bottom to be biologically inhabitable. A separate mechanical solids filtration connected to the bottom drain then biological filter before it goes back to the pond via GB-RAFT- Vertical/tower system, strategically distributed to the borders of the 6 hectare property down to the catch pond. The Pond is located at the center and the deepest part of the property. About how much more fish do we need to stock? 

No more. I'd start with what you have and give the whole system time to mature.

Thanks Averan! I will check the water chemistry again this weekend.

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