Feels good when a planted guy moves to marine
There might be many contradictory suggestions I may mention, but those are from my experience you may ignore.Size of the tank
You can go with 1feet cube, 2feet or 20 gallon tank or smaller size tank.. but smaller the size of your system the more it will tend to destabilize in future. Choose the largest possible water volume keeping in mind your budget and the asthetic you are planning to.
IMO choose a wide tank >= 18inch it will help you in scaping the tank to allow proper space for water movement. Another point is height, most of the invertebrates which we keep in our tank are collected from >18inc below water level, so it is recommended to atleast keep your tank 18 inch ht. It will also help to scape your tank. Sump
The most important part of reef tank is sump.
Size of the sump - The bigger the better. Try to built the largest possible sump you can accommodate in your space. Keep the design of the sump as simple as possible. It will help you in maintenance of the same in longer run.Design suggestion:
Keep the mechanical filtration part above the sump, the water from the tank shall pass through mechanical filter media to the sump.
Make the first chamber of the sump with bio balls or bathing scrubber
... most of the snails lay eggs on this.
Second chamber for skimmer, this is the heart of any reef tank. Keep enough space for a in-sump skimmer .SKIMMER
Many people may suggest you to go without skimmer for small tank, regular water change will do....... but its not practical. I have tried and studied with different set ups with skimmer WO skimmer, but belive me the better the performance of your skimmer the more stable your tank will be in longer run.
Third chamber for DSB - Deep Sand BedDSB
This is the place where beneficial critters grow apart from biological filtration. It should be approx 4inch in depth spread over minimum 1sq feet area. Water level over this chamber should not be more than 2 inches , it should be designed properly so that water flows over the surface of the DSB.
Fourth chamber for return pump.