I have a 72 bow front (a curved 75). I use a 20 long for my sump. I made too many chambers but you can see what it looks like in the build forums. I showed how I built it. My recommendation would be to make a two chamber. One large enough for a sump, and one refugium/return. My return section is too low so I have to top off my evaporated water every couple of days or else the return pump throws bubbles.
The 40 breeder is a nice size tank. Many members have built a full reef tank out of that. I went with the 72 gallon because it is 4' long and gives me room to play!
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