Aquaponics systems generally allow crops to be planted about two times as densely as you would in soil. Since each plant is getting all their needs met directly at the root-zone, there's no need for the roots to go searching for more water, oxygen, etc. This allows for much tighter spaces between each crop, as the roots are compact and small. As long as each plant is getting the right amount of airflow and light, they'll be alright with sitting closer to eachother.
You'll want to diversify your crops, as growing only one kind of plant can make them especially suscetible to diseases and parasites. Planting larger plants near smaller plants will give the larger plants more room to grow; while planting quick growing plants next to those slow to fruit can provide extra space for more mature plants.
