So I decided to switch to Prime from Stress Coat and noticed the dosages are different. If I take 15 gallons out do I add 1.5 ml of prime to the new 15 gallons I put in? and would it be okay if I added the full 1.5ml in the 1st bucket of water then go ahead and add the rest or the water? Hope this made sense

I always treat the whole tank, rather than just the water that I am replacing. If I remember rightly, 10 mm of Prime treats around 50 gallons of water so it does go a long way.

u do about the same. I treat for the whole tank for the most part, my 55 gal gets one capful (50 gallons worth) and my 150 gets 2 capfuls (100 Gallons worth)

a double or triple dose is safe for spikes, and treating a tank will not affect your fish during water changes. PRIME is the BEST!!!