That makes it stagnant. No, because it gets only a little ice in winter. It should be located out of sunlight and heat, which promote algae growth. You buy one. There are concrete vaults, plastic cisterns, a lot of choices.
Septic tanks are a different matter.
You want only something from a licensed supplier, and it should be a 2-compartment tank. The smallest size is 1000 gallons, overkill for a system with a constructed wetlands for graywater treatment. It sounds complex and expensive. Most people do the work themselves. I can do the design and help for an hour or for a day, and then let them do the rest.