Whenever you have a #-#-# thing, it shows how much macro nutrients it has, that is, its N-P-K values. N is nitrogen, in a planted tank mostly NO3 or nitrate. P is phosphorus, used by plants in the form of PO4. Lastly is K, or just potassium. Your first fertalizer (the 15-0-0 one with iron etc...) was pretty unbalanced with 15-0-0. Iron, zinc, copper, etc... are all mirco nutrients. Micros are important, but plants don't use them as much as macros.
Your second fertalizer is better, with an N-P-K of 0.6-0.3-2.4. However, the phosphate (or the middle number "P") is a bit high. Here a tip is, nitrate should be in a 10:1 ratio with phosphate, so there should be 10 times as much nitrate as phosphate. As you see in your fertalizer, it is 2:1. The potassium (K) is decent, it could be higher, usually 2-5 times as much as nitrate, but it has 4 times as much in this fertalizer, so it has good potassium. Did this one mention anything about traces (micros) or iron? If not, you would need some sort of trace fertalizer.
Now, about what you should be using. How much light do you have over your six gallon? If you have less than 2 wpg, very
occassional fertalizing is good. Over 2 wpg, then you could use it. I would guess that you don't have much from your last post, but exactly how much light do you have, what kind?, like flourescent, incandescent, metal halide-I hope you don't have halides on your 6 gallon, lol! Tell us more about your setup!
If you already used your fertalizer, everything in it is fine unless you poured the whole bottle in, so it is safe.