Myka, I built an acrylic sump 36x16x16 with a refugium last year. I'm not sure how many gallons it holds as there are different compartments each with different volumes of water but I'm thinking it is somewhere around 30 gallons.
Here is a picture I took today.
The water inside is fresh water. I wanted to hear how loud the pump was. With the door to the storage room in the basement closed, I can hear the pump running at the top of the stairs.