I was told to put 10-10-10 fertilizer in the water to help the Hyacinth winter better in the indoor pond in our basement. Will this hurt the fish in the pond? Also can granuals be used?

I don't like to use terrestrial plant fertilizer in ponds. In high amounts, it may harm the fish. In small amounts, probably not. It will promote an algae outbreak as well. I would stick to some liquid pond fertilizer, something like FloraFin which has potassium but not much of the nitrate, phosphate, etc. If you have fish with the hyacinth, their wastes also help feed the plants. I've never been able to keep water hyacinth alive inside over winter due to not enough heat or light. They really need a LOT of light, heated water, and some fertilizer but not too much as they won't be growing much indoors.