Your roots will develop healthier if you wrap that cup. They don't seem to like it when you can see them naked. In nature, they don't get exposed to light.
Fish is good fert,.. it's why we have thanksgiving in the US. I second the idea of transplanting and covering the stem,.. you get more roots shooting out where the stem is under soil.
Lastly,.. I think you could water a bit more often. Don't over do it with that organic fert, but water is a medium for the plants to take up nutes, and those yellow leaves and spaced out nodes might reflect a lack of nutes from underwatering.
Or I could be completely wrong.
Good luck.