Diving into brackish or saltwater can be pretty tough when compared to freshwater. There is more to balance (salt ratios), sometimes more care as the fish, invertebrates, and/or corals are almost all uniquely different in their slight care and comfort requirements, and with all of the different types of equipment - it can seem overwhelming. With all of the experience that you have now compared to when you first started out, what is one piece of advice that you wish that you could go back in time to give yourself?
The most important piece of advice that I would give myself if I could, would be to make sure that I gave everything just more time. I remember when I waited a few weeks to make sure that all of the equipment was running fine, the water parameters and levels were going good, and that my protein skimmer was working correctly without having any issues. When I added in my first fish (which were just two clown fish), I remember being so excited that I had finally gotten my saltwater aquarium going. Although from there, I only waited a week or two before adding a few more fish in. I remember when things started to get a little "too much" for me, in terms of when I had to do my first water change with fish. I remember trying so hard to get the salt levels just right with the new water I was adding in, making sure not to overdo it, while also trying to hardest to be super quick to get the temperature almost as an exact match. This may seem slightly simpler these days, but it was the first time I just felt overwhelmed trying to make everything perfect.
I had plenty of these experiences on the first go around where I just wanted to pull my hair out since I couldn't get everything perfect, and kept trying and try again. I know that it might sound like a simple thing, but giving yourself time to learn the process, and give it a few shots without the worry for everything to be perfect, it'll be easier to maintain the tank but also easier to grasp other concepts that let you be more precise as time goes on.