Good Advice above. I'd go for more flow in the tank first, then retest and add PH
buffer as needed. Your PH
might be all over the place because your tank is new and by adding fish as you did it may be still cycling. I would double check your levels at the LFS
But I wanted to add that for a 5 week old tank, that is a huge bio load for it. I would start a little slower on adding fish and inverts. Maybe 1 fish every 3 weeks.
Also for that size tank I don't know if you want to add a tang to it. I just don't think it will have a ton of space to swim. They do require a lot of space. So keep that in mind.
As for getting a skimmer and more LR
, I would wait until you figure out the first set of problems then you can move on.