Making your own sump is very possible and easy. If you put a refugium in the sump it will also give you a spot for extra rock and sand plus help with keeping nitrates down and the copepod population up (seahorses eat copepods). The other nice thing is it adds water volume to the system which adds some stability which is also important. I set up a sump out of a 55g tank for my 90g but you don't need that much. If you got a 15g long tank you would be set. You just need baffles to silicone into it. NOTE if you use an acrylic tank you should use acrylic baffles, glass tank should have glass baffles. Baffles are just dividers between the sections, there are typically three sections. The first is where the tank water enters the sump and is also where the skimmer is usually located. The second section is the refugium section. This section is where the live rock and live sand go along with some chaeto algae typically. The third section is the return section where your return pump is located that pumps water back up to the tank. So typically there are 2 baffle sets, one of the sets is made from just one piece of glass. The other baffle is actually made up of 3 pieces of glass laid out in such a way to remove bubbles from the water from the drain and skimmer. It's hard to explain and easier to understand if you look it up online, it's called a bubble trap oddly enough. I would seriously recommend doing the sump route as its going to be way better in the long run and will probably be cheaper than a canister filter. One last note from experience, with the baffles keep in mind water runs downhill. The baffle between sections 1&2 should be the tallest, then they get gradually shorter as you move towards the return. Also you don't need to use "aquarium" silicone. GE II 100% silicone for windows and doors- make sure it doesn't have mildew resistance and your set. If you want to check out my 90g build thread and you can see how a sump is typically laid out.
