I don't think anyone can answer the question unfortunately as you already tried everything. I am wondering about vitamins myself. I didn't read the review though.
But i am wondering if its really good to add to the water and give them vitamins.
I am working at the facility where we have fish tank. The guy who cleans it told me something interesting. He actually using flakes. He put flakes in the cup with dechlorinated water . Then he buys in the fish store only, garlic juice and vit c. I forgot though vit c or e. So he puts a few drops of each in the cup with the flakes. Let it sit for a few minutes and then feed the fish. This way the fish gets some vitamins. I think we also can use fresh squeeze garlic.