Hello and thank you for your inquiry.

Marine Depot sells a wide variety of sizes (flow rate) RO/DI systems.  You can view them there:

http://www.marinedepot.com/RO_DI_Water_Filter_Systems-FIRORD-ct.html

I would also like to link an article about setting up an RO/DI system that I think will help you some:

http://kb.marinedepot.com/article.aspx?id=11030&cNode=4O3V2M

RO/DI systems need to have a tap water source as well as a drain close by, so if this is possible while setting it up under your tank you could do this.  I personally don't like to hook an RO/DI system directly up to a tank just in case the float valve malfunctions for some reason this would cause the water to keep flowing into your tank.  Some float valves have a back up float and this would be your best bet if you chose that method.

You would need to split the output line so you could run one to the sump as well as one for filling up what ever container you use for mixing saltwater.

Hopefully that helps, but if you need more inforamation please feel free to contact us directly at customercare@marinedepot.com for more assistance.

Keith M.

Marine Depot Customer Service