What Tom just said, plus I would be adding some Seachem Stability - twice the recommended dose until things balance out. Good luck!
I'll say it again he tank is not cycled. Use some media from the 75 to seed the tank. Do smaller, frequent water changes. Reduce feeding, dose with the new prime/safe.
You could also filter the water through a mesh bag with some peat moss in it to lower the pH. Ammonia is less toxic at lower pH levels. Small 25% water changes and a cup of aquarium salt to the tank replacing a quarter cup with each 25% change. Add zeolite (ammo chips) in bag to the filter this will absorb ammonia the cycle will take a little but your fish will survive.
Ammonia is also less toxic at lower temperatures.
Buy extra safestart. And dont do any wc or add dechlorinator before dosing. I think you have to wait 24 or 48 hrs to dose safestart after using prime or any dechlorinator. Amazon has it for a good price. Feed very lightly. Dose for the whole tank you"ll probably need 6 bottles to be safe maybe more depending on how many doses it takes to get nitrates and even then it wont be mature so continue to feed lightly. 3 bottles would treat 225 (75 gal per bottle if you buy the larger bottle size) i always do extra doses. Ive tried stability and safestart worked much better for me.
Would also get safe it will last you longer and you wont have to worry about what your lfs is refilling your prime with anymore. Hope this helps but i would definitly seek/listen to the other members advice