That is a shame, but it wasn't a good idea in the first place
Stick with nano fish, such as sixline wrasse, clowns, gobies, some blennies, you don't want fish that are going to be messy eaters and really mess up your parameters since you are new to saltwater. I would also stop taking advice from your lfs, their job is to get you to spend as much as possible in their store. Meaning they already sold you a fish which had no business being in your size tank, now that it has already died guess what, you are gonna go back there and buy more fish.
Do your research first...then go buy fish, not the other way around. This will save a lot of heartache as well as a lot of money. There is a reason that a lot of us nano people have the same fish in our tanks, because they are proven to be able to live in such a small enclosed space.
I like your aquascaping but like you said you need a lot more lr, as for the lighting what about a used sunpod? You could get them in 70 or 150 watt and be able to keep anything