Vitamin A dosage?

Currently housing a pair of azurs. Although I have not observed them eating. In general they haven’t had much of a feeding response. They’re still new, I acquired them on the 2nd of this month. Additionally Their weight is healthy and consistent. I added extra spring tails, have dwarf purples, as well as breeding powder blues housed with them. I wasn’t all that concerned, until recently.
I saw the female trying to catch something right in front of her. Her tongue appeared to move in slow motion. The male doesn’t move around much at all. Is skinner than the female. But nothing alarming.
Wanting to know the last time they were given vitamin A, so as to not overdose. I called the pet store I got them from. Only to be informed that all their darts (breeding or not) are ONLY given rapashy calcium plus!

Upon learning they aren’t given any additional vitamin A. As well as behaviors observed. I’m convinced they’re vitamin A deficient. I immediately gave them rapashy’s vitamin A plus. I dusted the flies. Also putting a little bit in their water dish. The addition to the dish is because I haven’t seen them eat.
They are currently on a feeding schedule with rapashy’s calcium plus, superpig, supervite and vitamin A plus.
I have seen the flies take all their dust off. Before crawling around the enclosure.

My concern is, they’re not consuming enough, if any, vitamins. Before the flies have gotten the chance to wipe the powder off.

How does one dose rapashy’s vitamin A plus through the water? How much is too much? What should be my dosing schedule for added vitamin A? Any other supplement recommendations for azurs, that have only been given calcium plus?