My tomato plants started getting yellow leaves. Looked like a magnesium deficiency. Fish water has loads of nitrates and phosphates but can be weak in magnesium, postasium, and trace elements.

I used dolomite, which is a calcium magnesium carbonate that effects pH only slightly and they greened right up.

For Boron I use 20 Mule team Borax. Boron is a trace element. You don't need much. Go sparingly. Too much can harm invertebrates.

Postassium can be found in wood ashes, but be carefull with that. Ashes can really spike your pH. Add to the beds and not the water.