AP update - Death

[Devonian]

Sorry to hear about your problems and I have no experience of aquaponics myself, but I have kept fish, although I am certainly not expert. But from what you are saying the only things that I could think of are:

When topping up the tanks, did you do it all in one day, or was it done gradually over a number of days to help reduce the effects of chlorination on the tanks from the mains water?

What used to be stored in both the blue barrels and the fish tanks you are using, could there be any residual contamination?

I'm not sure where you are located, but down here the weather has warmed up again in the last couple of weeks and being in a greenhouse, I should imagine the water gets quite hot, warm weather (water) reduces the amount of oxygen in the water which could be a problem. When the fish were on their sides, where they gulping for air?