When you start an aquaponics setup the bacteria are colonizing the media. This is what we call “Cycling” the setup. The biodiversity is constantly increasing and a complex ecosystem develops day after day. The biodiversity is perpetually evolving with time and seasons but there is a “climax” generally reached after a full year of development. At this point a very wide range of minerals and nutrients are released into the water and available for the plants growth.
I generally recommend to plant young aquaponics setups with leafy vegetables such as lettuce, spinach, silver-beat, celery… Those vegetables can be classified as “easy growers” in the sense that they mainly grow leaves and leaves are generally less demanding than fruits.
Fruit plants such as tomatoes, chilli, egg plants, capsicum, zucchini, cucumber… require a complex blend of nutrients only available in “mature aquaponics systems”. In aquaponics each living creature is specialized to transform a specific element into another and this is the reason why mature ecosystem with a strong biodiversity offer a great blend of nutrients for fruit production.
To produce a fruit the plant need to first produce a flower that will then be feconded (generally by insects) to be transformed into a nice and succulent fruit.
This activity requires a specifically large quantity of potassium. It is indeed well known that potassium increase plants flowering and fructification. If your aquaponics setup is mature but the fruit plants are not producing much you can increase the quantity of potassium in your system by adding some potash into your water.
Less is more!
Keep in mind that a significant quantity of potassium will boost the flowering of all your plants and this include the leafy plants. I generally recommend to keep a large diversity of plants together in your growbeds so you will probably have leafy plants and fruit plants in the same setup. If you add some potassium in your water it will act on the leafy plants and will push them to produce seeds. When a lettuce produce seeds it grows is height and decrease the leaves production. Furthermore the taste generally becomes bitter.
A question of compromise…
The general idea of Aquaponics (as opposed to hydroponics) is that we are not working in monoculture but in a large and harmonious polyculture. Therefore the environment should not be specifically adapted to a specific plant but should offer the best compromise for all plants of the system. Be thoughtful when you add your potassium and only add small quantities at the time to avoid having all your leafy vegetables growing in height and becoming bitter.