I've used the blue Peter's 20-20-20 powders before, but only for vegetative 18/6 or 24/0 light periods.... When in mid-late vegetative cycle you can use heavy dosages of the 20-20-20, Peter's recommends a tablespoon for every gallon of water.

When u flower you want something with more phosphorus and less nitrogen, like 2-5-3 or if you were using Peter's 20-20-20 to veg for the first 4 weeks when u start to flower switch to a 10-30-30 ratio fertilizer!! Otherwise your buds will have more leafy material than actual buds!

I've since switched to Fox Farms organic and Advanced Nutrients organic. I would always recommend organic over chemical ferts.

Have fun and good luck