Your levels don't seem to far off. Ph is a little high but not by that much. 8.2-8.3 is ideal so 8.4 is not bad. Nitrates are a little high but this should not be too much of a problem considering the animals you have in the tank. Are you running anything other than carbon in the canister filter? It is ok to run carbon but try to stay away from things like foam or filter floss, or other things specifically designed to harbor aerobic bacteria. Adding more liverock or incorporating a refugium into your system may help with lowering your nitrates.
As far as additives are concerned you will want to dose a calcium additive in conjuction with an alkalinity additive to maintain an ionic balance. Also when dosing strictly calcium and alk you will also need to add some trace elements. A 2 part additive might be a good choice for you to start with as it takes most of the guesswork out of dosing since you add an equal part of each additive. Plus 2-parts also include trace elements. Just something for you to consider to make things easier for the time being. Your calcium and alkalinity demands at the moment are pretty low. 350ppm on the calcium is not that low and is actually close to natural seawater. Most SPS keepers prefer to elevate calcium to 400-450. If you find yourself constantly adding calcium but your levels do not increase then you likely need magnesium.
Sorry to hear about the yellow polyps. There is not much that can be done in that situation. At least the green polyp survived.
As far as powerheads are concerned, the tunze streams are great if you can afford them. If not then the seios are a good choice too.
Good luck with your tank!