If you find yourself in need of nutrients, I've found water soluble kelp powder to be a great amendment. I've also used actively aerated
compost tea to supplement nutrients and introduce new microbes to my system.
Eggshells are fine for aquaponics, although I've found they keep the pH a little too high for my taste. I'm starting to prefer running my system at 6.0-6.4. Seems the lower I swing my pH, the better everything does
Before we figure out what supplements work best, I'd like to ask about your source water. Are you using city, well, or rainwater? Well water, for example, is usually quite hard, and won't usually need added calcium or potassium (you'll find yourself going through more acid instead). Rainwater will need to have calcium and potassium supplemented regularly in some form.
I also would mention that lime juice is a "no-no" when it comes to aquaponics. We're building a system that thrives on bacteria and lime juice is anti-bacterial and therefore counter-productive.