Surface "oil Spill" After Fertilizing

B"H

hello
I bought API leaf Zone and found it's missing Nitrogen & Phosphorus so i got both of this from Seachem .
and i use recommended dosing, i didn't poured it to the tank directly but
take a water from the tank & mix all 3 fert's in a cup, and slowly pouring it back to the tank.

but after about 24 hours the tank top is resembling an OIL SPILL!
is that typical & to be expected from fert's?

also i noticed some of the neon tetra's lower rear fin tip turned a while back white on some of them.
but could that be the start of fin rot & related to the fert's chemicals i've used?
thank you.

best regards
Yaron