I suspect the biggest problem is ammonia. OP, do you have gravel in the tank? If so, how often do you vacuum it? I do partial water changes and gravel vacs each week, often 2x a week. Waste can really accumulate in gravel, especially with goldfish. I'd also be worried about leaving sliced veggies in the tank for 3 or more days, as the veggies break down it can foul the water. I leave fresh cut veggies in no more than two days, and if I have blanched it, less than a day. It will literally dissolve in my tank, oh what a mess, and if you have water quality problems to begin with that could make it worse. To keep my veggies weighted down, I have a heavy plant base that used to have plastic plants sprouting from it, and I removed the plants. I inserted a blunt screw threw it, and I "screw" a section of sliced cucumber or zucchini to the screw and voila it sinks to the bottom.