To build a fish tank sump, first of all, I recommend you buy a sump. I'm not going to recommend you make one, because you're probably setting yourself up for more problems than it's worth.
But if you're enthusiastic and experienced enough to try this, basically the needs of a reef sump, which is what you would build a sump for, it's just a vessel to hold water. It's also going to hold your protein skimmer, your mechanical filter, your carbon, your floss, so I would copy a regular reef sump that's out there now and just try to duplicate that.
Basically I would start with a fish tank, like a 30 gallon fish tank usually works well. It's 3 feet long, 12 inches wide, like 15 inches high, perfect proportion for a reef tank. You'd have the effluent from your overflow box dump in one end of this sump and dump into a micron sock which will filter out a lot of the particulates in the water. And that you'd clean once every week or so until it overflows.
Then you'd have a protein skimmer in the chamber that the water first goes in. The reason for having the protein skimmer there is to give the skimmer access to the nutrients before they have a chance to go through the rest of the filter system, and to also allow the fine bubbles to be released in the beginning of the filter as opposed to the end where the return pump will pump them back up into the aquarium.
So, you have your micron sock, your protein skimmer, you could also put a carbon bag there. And then you can put your mechanical filter like your filter floss or your sponge as a pre-filter to your main return pump.
Now, to section everything off most people use egg crate which you buy at Home Depot. It comes in two by four foot sheets. It's like ceiling light material. It's just white egg crate. You can cut it with a scissor or with any kind of saw. And you can silicone it using aquarium safe silicone to make barriers inside the sump. This will section off all your chambers. And then within these egg crate barriers you can put floss. You could put square sponges.
And that's the easiest way to build a fish tank filter. You can get into working with acrylic, getting the special glues, the saws, but that's a different video altogether. That's really for the experienced person, because if you glue it and it doesn't hold watertight it's going to pop two, three weeks from now. It's got to be braced right. You've got to use really thick acrylic.
So I recommend just going with a regular 30 gallon fish tank. And you can save yourself some money. It's actually pretty easy to do.