Haha, well making mistakes is part of the learning process. We've all made our fair share, even those who don't admit it. When I first set up my CO2 system, I nearly killed off all my fish cause I didn't remember to turn it off at night.... I immediately bought a timer that next day, but still. So it's unfortunate, but it really is a way to learn, cause if you're really learning, you'll never make the same mistake twice.
And good to know it's just the black frame.
Hmm, were you adding any sort of organic material or just straight ammonia to start your cycle? If there was nothing for the bacteria to feed on then the "cycle" never really began when you filled up the tank, so you would be reading zero pretty much the whole time until the bacteria find something to consume. So I would definitely keep a close eye on things now that fish are in. Depending on the pH of your water, the ammonia might not be a problem, but once nitrite begins to build up you could have problems for your fishy friends.
Carbon can remove some metal ions, organic contaminants, and oxidants like chlorine (from untreated tap water for example) from the water column. Carbon is useful or maybe even essential in a fish-only aquarium where there are no plants around to consume these metal ions and organic material, but with a well-planted tank, the plants perform similar functions as the carbon so it is unnecessary. Others have also reported "leaching" of formerly trapped material in old carbon filter media, but I haven't seen much on that other than hearsay. Though I admittedly haven't searched for it either. So basically, as long as you are properly treating tap water, then the carbon can typically be left out in the future.
Supplements like Flourish contain some of these metal ions (the micro nutrients) intended to feed your plants that if not immediately taken up by the plants will be stripped from the water column by the carbon in the filter meaning your dosing may not go all to the plants and they could become malnourished. I don't believe carbon will do anything to Excel, but I'm not sure about that.
The only time I would suggest putting carbon back in the filter is after performing a chemical treatment. Either for pesky algae with an algaecide or treating illness in the fish. After a successful treatment, I like to put some carbon in to make sure the chemical additives will be stripped out of the water to bring things back to normal again.
Don't hesitate to ask questions on this forum. It's been a great resource for myself and plenty of other enthusiasts/novices. I've found the people here to be generally pleasant and always willing to offer an opinion even when none was requested.