I've worked with PTU enough to know it is pretty insoluble.
Any advice is welcome.
The stock recristallizes with time, but goes into solution again upon reheating. I find prewarming the stock more convenient than having to weigh out this nasty stuff every time, and having to worry wether it will have gone into solution sufficiently well. Add 1 ml of stock solution to 100 ml of fish water when the embryos are 10 to 20 hours old. I would guess, given the limited solubility of PTU, that trying to get it into solution in Hank's might be a problem. However, for your purposes it may not do you much good.