Definitely want to upgrade your skimmer if you are going to start keeping sps corals. You may want to look into a refugium as well.
It is possible to go sumpless and run a closed loop and an external skimmer but if you have the space then I would recommend a sump. Having a sump will increase your water volume and increase your water surface area which will increase your evaporation rate to help cool the tank. Not to mention give you a good place to house extra equipment.
How big of a sump are you thinking about getting?
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