I've grown a few plants in windowbox shaped planters before and it works fine. For 18" of height You may want to forgo the veg and go from seed to 12 hours of light. Topping and/or LST will be necessary.

Seedlings don't like or need added fertilizers. Wait until week three unless they show signs (light green).

Earth worm castings is great in tea. Also add a little of the bio-tone.

The pH from the runoff is too high. Where is that from? Did you wash the coco first? You really want to figure this out, now.

No aquarium water for the plants. Put it in a compost pile, if you want to recycle it.

Good growing!