How to get weight up on a firewater flit bugs alive in 75 hard keeper 

You have to give more detail. Age? Teeth done? What is the current diet? Hay quality? On pasture ect... to many unknown variables to your situation.

I have very goid results with Tri Amino. It is fairly inexpensive. Also check for ulcers, check for worms, check teeth. Make sure you are feeding enough.