Pelvic fins don’t move that much I think they are mostly to stabilize the fish not propel it. If it is a disease what will help is if you look at pictures and videos of sick fish and read up on how to identify illnesses. I’d also suggest taking pictures of the fish you buy and if it dies or seems sick take more pics and post them here for help. Bulk reef supply has a decent video on YouTube about fish diseases that can be a good intro.
Having nitrites is interesting. Did you seed your tank of a bacteria starter? Might want To try that.
For filtration the most common HOB choice is probably the Aquaclear. You could do a 70 or even a 110. In the Aquaclear people put all Sorts of things - filter floss, carbon, phosguard, and bio media like ceramic tubes. Go to the build thread section of REEF2REEF and do a search for Aquaclear and I’m sure you’ll get some ideas! You can even modify the Aquaclear into a refugium for algae like chaeto to grow- there are threads and videos for how to do that here and on google, although maybe that’s something for you to consider doinh a little
Later.