Fish Fertilizer acid / base reactions
Cells of fish contain proteins, and proteins are made of long chain of amino-acids, linked together by peptide bonds. In the presence of an acid (like phosphoric acid in your experiment or hydrochloric acid in our stomach), the peptide bonds are destroyed and the free aminoacids are released, so that they can be immediately assimilated by living organisms, like roots of plants if these acids are added to the ground as fertilizers.
But the phosphoric acid is not destroyed in the process. It still remains in the aminoacids mixture. As most acids, it is a corrosive substance that must be neutralized before being disposed in the ground. This neutralization is made with KOH, by one of the reactions $$\ce{H3PO4 + 2 KOH -&gt; K2HPO4 + 2 H2O}$$ $$\ce{H3PO4 + 3 KOH -&gt; K3PO4 + 3 H2O}$$ Whatever the reaction chosen, phosphoric acid is now transformed into a potassium phosphate, which can be assimilated. The advantage of this process is that now the fertilizer contains amino acids (bringing Nitrogen N to the soil), plus a salt bringing two important elements for fertilizing a soil : phosphorus P and potassium K. The three fundamental elements (N, P, K) for the agriculture are present in this fertilizer
To go back to the use of KOH, the equation shows that the amount of KOH to be added is twice or three times the amount of the initial phosphoric acid. That is why you have to add a huge amount of KOH to your mixture.