**A permanently primed siphon can equalize the two tanks.** Run a hose up over their rims, with **both ends submerged**, preferably near the bottom. Assuming both tanks are vented to the atmosphere, water will flow from the higher water surface to the lower one until their surfaces reach the same elevation. It can flow either way as water is added or used.

To set it up:

1. Fill the hose completely with water, removing all air. This is called *priming*. [FAO siphon instructions](https://www.fao.org/4/s8684E/s8684e09.htm).
2. Seal both ends temporarily, place one end underwater in each tank, then open them while submerged.
3. Secure the ends so they stay below the lowest expected water level. Keep the hose’s highest point close to the tank rims.

Once the levels equalize, flow stops, but the filled hose remains ready to transfer water when a difference develops.

**The limitation is loss of prime:** if either end becomes exposed, or air accumulates at the top, the siphon can stop and need priming again. An unsecured end rising above the water is a common cause. [University of Washington demonstration](https://www.ocean.washington.edu/research/gfd/oc512-gfd1-archive/expinfluids.1a.htm).

The levels will also differ slightly during transfer: a difference in water level drives the flow, and a narrow hose may transfer water too slowly to keep up with use. A bottom connection is the more dependable permanent arrangement.

Being on a three-storey building makes little difference; the relevant height is the hose’s crest **above the tank water surface**. Agree on the connection with the owner, since it will let either household draw from both tanks.