IMO the canister filter needs to be cleaned more then monthly. fish only tanks can tolerate a higher level of nitrates but it is stressful on fish and to get their maxium lifespan and health nitrates should be kept to a minimum. the reason i am talking about nitrtates is because canister filters ( or any filter for that matter ) traps detritus and debris causing spikes in excess nutrients. i am honestly suprised you havnt had bad algae issues yet, im guessing your using RO/DI water. saltwater tanks will do great with sand, live rock, good flow and a good protien skimmer as well as weekly/bi-weekly water changes.
i think that is best that your skipping out on the tang, the angel as ive stated can possibly work granted it doesnt show aggression. if your planning a reef down the road the angel will most likely nip corals. i have heard people succesfully keep one in a reef set up however its a big hit or miss usually leading to them nipping away at polyps. before moving onto a reef tank i advise you to read as much as possible to insure your success at keeping corals not to mention if you figure out how to do it right the first time around you'll save alot of money. search for reefing clubs in your area. my clubs yearly dues are $12 and it has well paid for itself over and over again. ive met great people with the common interest, learn daily and was able to pick up equipment for fractions of the cost.