I am not going to get on your case about a bunch of stuff, but just suggest you take this as a learning experience. Take some time to read up and browse the forum for a few weeks. From the description of your setup, it sounds like you have been getting mixed advice. What happened does sound like a low-oxygen event, so circulation is important to fix. A 55 is really too small for tang(s), so probably stick with clowns, gobies, maybe a fairy wrasse or something until you are ready for a tank upgrade.
Are you just doing fish only, or coral as well? If you are doing coral, Nitrates at .25 will be a problem. If not, it still isn't ideal and you should consider steps to keep it lower. A canister filter also is not ideal, but can be worked around. You should consider a skimmer.
Things can get expensive, so I would suggest you take the time to consider what your long term plans are for the tank and spend some time reading up. It can be addicting/time consuming to spend time here or reading, but you will save yourself a bunch of problems, money, and have a happy tank for your efforts. If you have a thought or an idea on your setup, feel free and ask around. You will get a bunch of opinions, but the answer is usually in there somewhere.