Hard to imagine an 8' fig tree having roots invasive enough to break into your cistern. Yes, you should move that tree, at 8' there is a very good chance at survival. Gotta see where it is that you and that tree are located. In the Northern hemisphere we are about ready to go into fall then winter. Dig a trench about 2' diameter, a foot deep around this tree. Fill with straw. Keep moist until your soils start freezing. In the spring, you should be able to pop this tree out and transplant.
Newbie plants/trees need regular moisture until well established...2 years is good. If not, they will grow roots to find any moisture they are able. But trees should never be planted near a septic system and this tree is young enough to move and perhaps still save. By digging a trench now the tree will have time to develop feeder roots WITHIN a smaller root ball before being transplanted. Do not fertilize if your tree has had at least one decent fertilization this year. I would buy some MYCORRHIZAE to assist with root development/uptake. Keep watered but NOT OVERLY WATERED. In the spring, you can move this tree to a new spot with a bit of MANLY help and a tarp. Make sure to NOT bury any deeper than the root ball. No bark of the stem should be compromised with soil or mulch. Don't dig any deeper than the rootball as settling would lower the bark into contact with soil/mulch. Before any planting, digging, construction, you need to know where your underground utilities lie. All home owners should have an 'as-built' included with their mortgage documents. This shows footprints of existing buildings, septic tanks, septic drainfields, electricity, cable as well as irrigation and water lines. In the U.S. we have a city/county service called 'locate'...give them a call and within 24 hours someone comes out to paint all electrical/cable before digging. Otherwise, you need to dig down to find your septic tank and make dang sure you find 3 corners!! Grin, hope this helps...