Can you isolate your FT. turn 2 valves and send any GB water to just the sump?
If it is the problem of sprays, maybe you can dilute the problem to a minimum.
I have never had your problem because I am inside with the fish. But this is my idea. It is up to you what you want to try.
1) I would get the FT off line ASAP. Drain 30% of the FT water and then do a running change of water if I could. I make a mark at the level, turn on the drain and the fill and let it drain and refill for a day. This is not at full blast, I only use a 1/4 inch hose for the fill so it is rather slow. My water comes in colder then the tank so I place the fill line near the air bubbles or in the filter return to mix it. I would add a temp cheap floss filter if needed to clean the tank. A bucket over the FT with floss inside and a hundred small holes in the bottom and run it from the FT pump. On our outside Sell fish to eat tanks we used a large plastic dish drainer They look like a giant Net cup and can also be used for sifting gravel. Cheap. The goal is to save the fish right now and give them good clean water. Forget the plants for a moment.
2) drain and clean the sump. rinse it out well. Refill it at least twice and drain twice. Scrub the pump and all walls. If it comes apart real easy I would do it and clean the parts inside also. A lot of impellers get caked after time and it impedes the flow. If you have to break seals and don't understand it, soak it in hot water and see if a bunch of crap comes out when you start it up (not in the sump) When ready just run the sump to the sump for while to get it turning and degassing. Add air if you can.
3) If you can isolate each GB, I would. I would also drain and refill them several times. Even using a garden hose on the top to spray them down. When you think you are ready, use the water out of the FT drain to refill them the last time and keep them flowing to the sump, by-passing the FT.
4) If you move 30% of the FT water to the beds, even if by hand, the fish water should keep clean and free of high levels, ammonia and Nitrites. Use clean make up water and let the GB drain to the sump.
5) I would keep the FT off line like this till I was sure the fish are no longer going to die on me.
6) Since the GB and Sump are now isolated from the Ft, you could try to put a fish in the sump and see if it lives for a week. If the water has been cleaned and harmful pollutants removed it should do fine.
7) If it dies in a day or 2 then you have bigger problems to task.
IMO every AP FT should be able to run in isolation if needed. A couple of cheap ball valves can save a tank of Fishies. The moment you saw them going belly up I would have turned the valves and isolated the tank. All tanks should be prepped to be able to go it on their own.
9) Any GB can be watered by hand if need be. Think of it as a long term power shortage and your backup would only run the FT air and filter. If only air then have an air lift to the filter.
10) Keep as much emergency water somewhere that is kept or treated to be able to use in a minutes notice. I have enough of it so I could always bucket my fish in different water. You would need a collection of buckets with many AP fish or just toss them in what ever is holding the water and add air stones. On my hold water I always bubble it for 3 days then cap it off. I use the 30 liter plastic jugs. In the case we ever get into survival mode it is the water that we would also drink. So it has a double duty. 30 liters is about the max I am willing to carry if there are stairs around. Do not mix this water with the ft water. I net the fish, Pull them out of the net one at a time by hand, shake the water out of the gills and toss them into the bucket. Never lost one doing that. It is the same way I put them in. I never ever mix waters. I have learned that lesson the hard way.
China story: When they deliver several thousand eating fish to the market, they use a water truck that looks like a septic truck to me. It has a hatch and a large 6 or 10 inch valve in the bottom rear. They pull up to a stocking pond and open it up. Fish come flying out. when the water level gets low, some guy with boots on gets into the tank and shovels the remaining fish to the drain. any fish too large get tossed in the air to the ground where another catches it in a net or by hand. After the restock, the vendors will come by and pull what they need to sell. All of that just takes a few minutes and they make many trips a day. People eat a lot of fish here. Take a look on Google Earth and you will see a fish farm where ever there is a supply of water. One we saw yesterday has 18 ponds. What an AP system he could have, yet they waste all the water down the drain.
I hope that the stuff that entered your system is water soluble and you can dilute it down to almost gone. If it is like DDT then it will never go away. Maybe you can ask him what he used and call the company for advise. There might be something simple to add to make it neutral. I don't think you will ever need to toss out all of the media but it could be a very hard job to get the contamination out of it. Cross that bridge only if needed.
While doing all of this, look real well for any other objects/liquids than might have fallen into or placed in your GBs. My kids friends will sometimes toss crap in the Ft just for fun. Hey do fish drink milk? It is now off limits to them to even touch it. And it has a 100% cover.
There are many APers here that have many years of running AP problems, I am sure one of them will chime in and also help out. I am still rather new to this AP stuff but I can call of 40 years of mechanical engineering to solve simple water contamination problems. Liquids are liquids and can be purged.
Don't worry about the GBs being off line. I took My test 1 off line now for 3 months. Just top off with FT water and it is actually doing better. My guess is more air to the roots and I have the drain level too high.
Good Luck. Save the fish!
Backyard Aquaponics
