Hi,
Sorry for your loss. First rule for success...DO NOT ASK THE LFS FOR ANY ADVICE EVER! go to the LFS knowing exactly what you want, or need.
I just kind of skimmed over the entire thread...nitrates don't really bother fish, even into the 100s. Nitrates affect inverts (over 40) and corals (1 to 20, depending on the type of coral).
You should quarantine every new fish for at least 4 weeks, so you can be sure it isn't carrying parasites and it's free of disease. You ABSULTELY DO need a power head, I can't believe the fish store told you differently. The wave is the life of the ocean, and your SW tank. SW has less oxygen then FW, so (for a larger tank) point a power head to the surface so it makes it look like it's boiling....that creates good gas exchange.
Ideally a PH that disperses the water in a wide wave like motion, the Maxijets for example, only create a jet stream. The Koralia type (looks like a fan in front) are much better to do the job of creating a wave motion. In your case, with a 10g ... A simple air line (NO STONE) attached to a rock via a rubberband, would allow for enough surface movement and create the needed good gas exchange for you.
You should only do a 3% water change unless there is a serious problem.
Any tank under 29g is really best left to the experts. There is no wiggle room for error on a 10g, and to be honest a 55g to a 75g is the best size for a starter tank. If you can afford it...get a larger tank, and use the 10g for a quarantine. Fish are stressed when placed in a tiny tank, the clowns need at least a 20g tank, and 30g is preferred. So as to why your fish was stressed, and you think that's why it developed "dropsy" ...blame the tiny 10g tank.