Advice? 20g Sump Design

I would make your media shelf deeper so it can hold more media. 1/8 is perfect. You can go 9" and then use egg create if your skimmer needs to be raised up.. Other than that it looks real good! If your DT is bigger then 55, I would go to a 40 breeder. Wait for the Petco dollar per gallon sale.

 

Looks pretty good!

If it makes you feel any better, that new sump of mine is actually the first sump I've ever had which utilizes baffles. These finer details aren't going to make or break anything.. they are all just "nice to haves" at this point in my opinion. 1/4" glass should be adequate for baffles in a 20g sump.. 1/2" would be extreme overkill. You're right on with 1/8th inch gap on the glass (as long as the baffles are glass rather than acrylic).

In regards to your height on the first baffle you're going to essentially set the height of the first chamber with that baffle. You have two things to keep in mind here.. backflow volume and skimmer depth. Make sure your empty space in the sump can accommodate the backflow in the event of a power outage (I determined how many gallons would backflow into the sump and solved for the appropriate empty volume). Since your first chamber is going to house the skimmer you may want to set it at an ideal depth for your skimmer (since many are sensitive to water depth) or you may need to build a shelf later for the skimmer to sit on.

 

Oh... and you also may want to research a little on whether you want your current bubble trap design (over, under, over) or a the under, over, under design. I went with the later. There seem to be pros and cons with each. However, since your overflow is already going over the first baffle which should keep most of the bubbles out of the lower regions of your second chamber, I think you're probably better off switching your bubble trap around. If you would have had your bubble trap situated right after your overflow I would have a different opinion.