collecting rainwater in a concrete septic tank

A risk of water supply interruption has arisen, so I plan to create a water reserve. I’m going to install a 300-liter tank in the basement. I had an abandoned septic tank; I dug it up and found it to be in excellent condition—it consists of two chambers. The first chamber has a concrete base, two concrete rings (1 m x 2 m), a concrete lid, and a brick access shaft. The second chamber consists of two concrete rings (1 m x 2 m) with a concrete lid and a brick access shaft, but no base. The soil is sandy, and the tank is located 7 meters from the house. There is a sloped overflow pipe connecting the two chambers. I want to divert rainwater from two downspouts into the first chamber; if there is a heavy downpour and the first chamber fills up, the overflow will direct the water into the second (drainage) chamber. Now I need to figure out how to divert the water from the downspouts and how to plaster the concrete shafts.

My main concerns relate to how the septic tank was utilized in the past. If, for some reason, it may of ended up getting chemicals/sewage/ick in it in the past that the porous concrete may be holding some of that. Depending on what you want to use the water for, you should consider it may not be potable after being in the chamber.