Need Sump Ideas

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I'm looking to upgrade my tank a little. It's a 37 gallon cube.

Here's a brief overview of what I have.

30 gal. cube
Aqualight Pro 150w mh + 1 55WPC 10K + 1 55W actinic PC
Remora pro w/ MJ1200 HOB skimmer
Fluval 205 cannister for Carbon and Rowaphos
Seio PH's (2): 1600gph total

Here's why I want a sump.

Temp tends to fluctuate more than I'd like 79.5 in AM - 83 PM

I supplement morning and evening with 2 part C-balance. I add it directly to tank and get substantial pH fluctuation. PH fluctuates between 8.o before and 8.2 after addition of 2 part.

I would like to add a small refugium compartment for Cheato with reverse cycle lighting to reduce PH swings.

I'm hoping that increasing the total amount of water in the system will reduce some of the fluctuation. Also having some water away frrom the MH lights will reduce some of the heating.

Any suggestions on a proper sump. I'm thinking of using a 10g aquarium. I can fit something about 22" long in the bottom of my cabinet if I nix the Fluval.

I might also add a closed loop to eliminate the Seio's and maintain my flow. I like the Seio's because they move a lot of water at relatively low velocity.