IBC tote water level alarm?

So last night we captured 700 litres of fresh rainwater. Most of it went into one tote but there was room so no biggie. I opened the valve on the other tote this morning to equalize the levels and that worked like a charm. But it made me realize 2 things. 1.) We need more totes. Our roof is only 1/3 finished so we're only capturing a fraction of what we will be when it's all finished, and 2.) It would be nice to have some kind of alarm that let's us know when we're getting full.

Has anyone got any suggestions for an alarm?

A float could easily trigger an alarm.  You could also modify a fuel gauge that you could read inside your  home.

What if it goes off in the night? Why not cut a hole in the top and use the overflowing water in a pond?

You haven't told us enough about your system.

I can imagine a need for an alarm, if you have the tanks in an interior space without a plan for excess/overflow water. (other than manual shut-off/diversion) This is a brittle solution, since it requires the alarm to work, someone to take action, and to be present to do so. You may also have a liability of a flooded basement? garage?

I'd lean towards some sort of passive design, as Hugo suggests, that can accommodate excess water without trouble or intervention.
Link your tanks together so that they either fill all at once from the bottom, or in a cascade... overflowing from one into the next, without needing to operate a valve (although having valves is a good idea for isolation in case of damage/repairs)

On the other hand, some sort of level indicator could be useful. To know when the tanks are full/low, when/if you might ration your use, or as an aid for noticing a problem (leak/blockage/break in the system).

A sump pump alarm or boat bilge alarm. 12 volt bilge pump and battery or solar, a sump pump alarm is usually 110 volt. Is it just an alarm you are looking for?

Even the humble flush toilet has an overflow that works passively, just in case the float valve fails.
With huge volumes of water captured fro the sky, over\flow becomes even more important.

You can run a pipe from near the top of tote A, to tote B, so that tote A overflows into tote B. you can also setup your system so that the top of tote B overflows over flows to somewhere else safely and passively.

My domestic water is stored in tanks in the highest part of the stairwell, which is built of rammed earth (without cement mixed in -- it's like cob), so overflow prevention is very important to me.

The passive overflow pipe should ideally be bigger diameter than the pipe that feeds the tank, in case the inflow is coming with speed and pressure, but the overflow just trickles out.

In my situation I don't want the overflow to happen frequently (It flies to the ground 2 stories below, splashing onto the earth walls and wasting water), so I ordered an overflow alarm from Amazon.in. It is attached to the AC supply which is fine for me because water only comes into the tanks when there's AC power for the pump. For a rainwater tank that fills most quickly during intense storm events that could cause power cuts, then maybe a battery-powered one would do you well.

I put the electric alarm just below the overflow pipe because overflowing is undesirable, but it would be easy to change. It's just two little thingies hanging on wires that you dangle down at the level you want the alarm to go off at. If I get really sick of the loud Lambada tune the alarm plays, I might direct the overflow water better, and lift the sensors up to the overflow level.