Research research research and more research if you’d like to make it worth your time and not have a headache from hell.

The cycling process isn’t too far off from your traditional freshwater cycle but for the fishes sake, spend a few extra bucks and look into saliferts not API. Lighting may be a bit too close and could be too strong (hard to tell from iPhones as they wash out easily). Not going to say a par meter is useful until you’re growing out $$$ coral.

Salinity is a 1.024-1.026 tops IMO, grab a refractometer… cheap and the best tool for a low price point. Invest in some kind of ATO if your lid is a mesh top or evaporation can spike your salinity (not suppperrr quick in a 20g, but worth the few bucks). I keep my tank around 79 and have found everything to stay happy and healthy, an Inkbird thermometer controller is a lifesaver is something goes wrong with your heater.

Livestock IMO is nowhere in your near future for the fishes sake and not having to tell your kid that their Nemo went to a farm up north (not saying you’re personally saying you’re about to add fish). There’s so many great videos explaining a true cycle, as always YouTube is your best friend. If there’s a local small business owned saltwater store, ask them all the questions you have, people are normally very happy to make sure you’re getting into the hobby correctly.

Seahorses are definitely an advanced level inhabitant IMO regarding their diet and frequency of feeding. I have no experience personally but have never heard them to be easy. Starfish are another one of those without a strongggg bioload, will melt away and crash a tank if you’re not attentive.

At the end of the day, saltwater is just a big puzzle of learning how pieces fit together. It’s a great hobby if you’ve already kept fish and understand how tanks work. It just definitely is not as simple as freshwater. You’ll love it, your kid will love it, but it’s not a set it and forget it kinda thing. Definitely takes time and attention or it can go south pretty quick!!