Aquaponic Gardening

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I work in a restaurant that has a 800 gallon NFT aquaponics system inside the restaurant. We have been in operation for 4 years, and about a year ago our nitrates skyrocketed to over 160 ppm and we haven't been able to get them back down. We've just purchased a lower protein food for the fish, which would go from 40% protein to 28% protein, but I don't have any data on this yet. We've done massive water exchanges, but it doesn't seem to impact the high level of nitrates. We've also added bacteria to the system to help the plants take up nitrates, which seems to help for 2-3 days, but not long term. I think we might need to increase plant density, but in the space that we have and in a NFT system it doesn't seem possible. Considering that the system was pretty robust for 3 out of 4 years, and now we're seeing this issue, I'm not sure what changed. The fish seem healthy, but I'm starting to see stress in the plants, like maybe they aren't getting enough calcium? The pH is right between 6.6 and 6.8 and our nitrites and ammonia levels are at 0. Being indoors, the water temp is almost always 75 degrees F.

Basically my question is if anyone has seen this issue before and if there are any troubleshooting ideas for reducing nitrates that do not involve massive water changes. Thanks.

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