This is a very big topic with hydroponics but it becomes a trickier one when you have fish in the system. You can't just douse your plants with fertilizer because it ends up in the fish water, which may or may not be a good thing.
In an ideal system, your fish tank water would be heavy in the components that your plants need (nitrogen, phosphorus, potassium, etc) so that fertilizing should be unnecessary. But when is any system running at ideal levels? If you've been testing your water and finding that it's low in any crucial element, you will want to start looking into fish-friendly fertilizer.
Sticking to the main minerals you'll need, here are some natural options for adjusting your water levels:
Nitrogen (N) - If you are low on nitrogen, you basically need more fish. With an adequate population, you'll never need to add more nitrogen. When your fish are very small (at the start of your growing system), you may need to add some seaweed extract or a general fertilizer intended for pond use. Nitrogen is the main component to help with leaf growth.
Phosphorus (P) - Bone meal (liquid, powder or actual bones) is the usual way to add extra phosphorus as well as calcium to your water. It's phosphorus you need to get lots of flowers and fruit in your plants.
Potassium (K)- The old-school method for bringing in a little potassium is to bury a few banana peels in your grow beds or pots. When they start to really get brown, take them out. For something a little more controlled, you can use various purchased potassium-based fertilizers. Potassium is a general-purpose type of mineral that just keeps your plants vigorous and healthy overall.
There are many brands of aquaponics-specific fertilizers you can check out. Unlike conventional fertilizers, they tend to leave out the nitrogen and are perfectly safe for fish.
A popular approach for natural fertilizing is using a tea made from worm castings (aka worm poop). You can either buy the castings or keep a few buckets of worms yourself to harvest their organic waste. Either way, you can soak the castings (they look like coffee grounds) in water to make a very nutrient-rich and natural fertilizer.
You can see more about the complex world of nutrients in the section on water quality.
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