Awesome, awesome, awesome!!!
I have a sponge and a power.Awesome, awesome, awesome!!!
Here's the bad news... you need a lid. It's not just gobies that are prone to jumping, but most saltwater fish in general. Given enough time, any fish might jump, and it feels like the risk increases with smaller tanks like you and I have. A 23g huh? Unusual. You can DIY a lid pretty cheap using parts from the hardware store.
What kind of filters do you have? HOB's aren't a bad thing, but generally hard to clean filters like canisters are a big no-no in saltwater tanks as they hold onto detritus and become nitrate factories. I know you're planning a FOWLR so don't need ultra low nitrates, but filter maintenance is something that needs to be done much more frequently in saltwater tanks, or you'll just end up with unsightly algae all over the place.
You'll also need rocks, sand, saltwater test kits, a refractometer for measuring salinity, and a powerhead or two. Hydrometers are garbage for measuring salinity and refractometers are cheap online. Flow needs to be about 15-20x your water volume, and I don't imagine you'll get that much from filtration. Your cycle will live in your rocks, and that bacteria needs well oxygenated/fast-flowing water to survive.
You also need to think about how you'll provide saltwater for the tank. Even with a FOWLR, I would not recommend using tap water. Ocean creatures are much more sensitive to small ammonia spikes you might get in the tap, and if you're like me, you wonder what else is in your tap. I'm in the city and have pretty old water lines I think. I'd be afraid of heavy metals that may be low in the tap, but over time could build up in a tank. I've seen crashes from things like copper and even tin in the tap, so...
You could buy RODI or distilled water and mix it with marine salt yourself. This would mean you'd need a spare pump and heater to mix water and bring it to the correct temp prior to a water change. You can also buy saltwater from the LFS, and this feels cheaper than investing in your own RODI system in the beginning. I would argue that the RODI pays for itself over time though.
Whether you make or buy your water, always, always, always check salinity before using it for a wc. There is a horror story here by Slug who bought water from a trusted LFS and used it before discovering that it was freshwater. It caused a complete loss of all life in his reef tank. I'm grateful to Slug for sharing this and allowing us to learn from it... refractometers are easy to use and it takes just a minute.
Edit: Oops! Meant to add that there are many nano tank-bred fish options. Clowns of course, pistol gobies, several bleannies and cardinalfish just to name a few.
power filter; internal filter powered with a powerheadYou won't need the sponge filter for the FOWLR as the rocks in the tank to support your BB colony. You don't need to buy live rock though, as dry rock will cycle and become live in your tank. Cycling is certainly quicker using live rock, but there's also the potential for pests like aiptasia. Still, lots of folks choose live rock and you can also use some live and mostly dry to save on costs.
I suppose you could use the sponge if you want it for mechanical filtration. It will need to be thoroughly cleaned with each weekly water change, and it will not help much with flow requirements.
Not sure what you mean by 'power', but I am honestly not experienced with anything but HOB's and all-in-one's that contain filtration within the tank.