need a skimmer for almost 300gallons

Ya, total system volume. Overall... you can stock a tank for fish based on total volume cause it is all circulating and filtered..... obviously you can't have a 500g volume system with a 90g display and fit that many fish in it... but generally it isn't that bad for most.

Most will stock to their volume so you should have a skimmer that will handle it. Your 1.5X display would work in most cases. I just go by heavily stocked system volume and then say 1.25-1.5x that.

 

Becketts are noisy and finicky compared to a needle wheel. They work fine, but I have just thought needle wheels have more to offer. Becketts use a much bigger pump too so more heat and power used.