I run my tank sumpless and skimmerless, but I wouldn't recommend it versus going with a sump. I'm used to being sumpless... have been for years and I make it work. The nice thing about a sump, IMO, are several:
- Added water volume being the biggest thing.
- Easier access to equipment... I hate it when people say it's a place to hide ugly equipment. If I had a sump, it'd be pretty and organized. I am just a bit OCD
- Bigger equipment! Things like skimmer, reactors, etc. It's always better to go bigger than smaller on most things, and a sump can let that happen.
- A sump is a nice place to have a temporary time out place in conjunction with a temporary frag holder. If a fish or other livestock needs to be isolated.. bam you have a place. You can also place frags down there without interfering with the main display's look.
- Refugium was mentioned.. though honestly I wouldn't run one, just a personal preference.
- Water changes have a slightly less stressful effect on the tank, when using the sump to drain and refill. Also just easier to do a water change in general, with a sump.
That's just off the top of my head.
My tank as a reference:
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