You are correct it is not yet time to use the fish stuff.
What was the mix ratio you used of superthrive?
ST has a LOT of nitrogen in it, and the shape of those leaves indicates N overdose.
WH is right, a transplant into nice fresh soil would do wonders for that plant. Allow it to get a bit on the dry side and it shoudl slide right out of the pot without too much disturbance to the roots.