Dang, that’s rough. Glad you figured it out though! Are you going to put a mesh filter in front or just rework the whole system?Well I found the problem...
For a recap I have a 1500 gallon + system made up of two large tanks. A 450G mixed reef upstairs and a 750G FOWLR in the basement. The sump, quarantine and frag tanks are also in the sump room. The 750 has one plank feeding pellets and the plank sits right on the tank, no problems there. The 450G is feed by the plank dosing into the sump right in front of the return intake. This necessitates a long piece of PVC on the plank, approximately 10".
one day while cleaning I noticed that there was food coming out of the top of the plank PVC. I took it apart and discovered the entire tube was full of ROTTING, nasty food. It was black and stinky. I cleaned it out and did about 2000G of water changes over the next couple of days. The pump at the bottom was operational. I could find no explanation what went wrong.
I thought it would be a good idea to reinstall without the pump as the PVC dumps into the sump and there is a good amount of suction pulling the food into the return. Everything was fine for a while but I had to leave on a business trip for 10 days. I get back and there are several more dead fish and once again the full PVC is full of rotting food, even though it was dumping into an empty PVC tube. So for now the plank is removed from the tank, as I really have no fish to feed any more. I just thought you guys should be aware that this can occur.