This is a wonderful setup - but I think my favourite part is the aquaponics return. Very nicely done and I didn't notice it until you pointed it out. Bravo!
Its an excellent setup. You could consider making some bcb baskets and put it in the last chamber for additional biological filtration and once the bcb baskets matures and develops anoxic bacteria, it will even breakdown the nitrates.
I like the set up. Would take the air stone out of the pond and use it to aggrevate the dead spot. The water flowing back into the pond is aerated anyway from the airstones in the filter system.
I mean itโs not a thing of absolute beauty but itโs ingenious and the fact itโs completely DIY, makes it extremely impressive. The aquaponics system for the plants was also a great idea. Iโd love to see you rake another shot it some day to improve on what you learned with this build. Great job though. Super nice ๐๐ผ