You could start with a 1lb bag. It will last quite a while.

It's not a substrate additive so don't pour it all in your tank. This is something that you dose daily.

You will mix 1/4cup mix with 500ml distilled water. Gatorade bottles are perfect for this. After mixing, store in the fridge so it will keep longer. For my 20s, I used 3ml. I added it to the tanks in the morning prior to the lights coming on.

My routine is to dose for six days. On day seven, you do your normal water change and do not dose the tank. Start the cycle over again the next day.