Buy Monopotassium Phosphate (kH2PO4)
Potassium Nitrate (KNO3)
Dose according to PlantBrain
I have an 80 gallon. Starting with 50% water change, I dose
3/4 tsp. KNO3 and 1/4 tsp. KH2PO4
Next day, 1/4 tsp. Plantex CSM+B
Same alternating for next four days, Saturday rest, start over Sunday after 50% water change. You will need to figure out how to cut my dosage down for your size tank.
Hope this helps a little.