Could a gasoline's octane level affect radiator performance?

So I have a '96 Honda Civic VTI. I've been using a 95 octane gasoline from Petron, then as advised by a friend who has a similar car, I switched to 97 octane gasoline from Seaoil.

After a few weeks I made the switch, my car had several problems - hard starting, variable revs when going above 3000 rpm, and the most peculiar one is that the water on my radiator and reserve is constantly draining, regardless if I use the car or not. If I park it for a day, the water reserves are drained, faster if I park it outside, on direct sunlight.

Now I went back to using 95 octane from Petron, now the hard starting is gone, revs are good and I haven't refilled my radiator in 2 days now.

Could there be a connection between th issues I had with changing the gasoline?

There's no connection.

The grade of fuel (even if it's wrong) has no effect on the water level in the radiator. If the level is dropping, it's leaking out some place. Possibly a head gasket problem if it's also affecting how the engine runs.

No. Get a mechanic to fix the leak in the cooling system.

I can't think of a real reason why exactly that would cause the problems youre having. 95 octane is pretty high test stuff use to use in a stock Honda, 97 I would presume is way too much. High octane gas is more resitent to detonation and can therefore be used in high compression engines. On a factory low-med compression engne however, you're really just wasting money as the engine was never designed to run that much octane. Spark and fueling tables are set up from the factory for 87/89 test, higher octane is going to change the ignition properties of the a/f mixture and most likely cause some of the problems you described.

What doesn't make sense is the coolant leak. There's no reason that should be happening just by running high octane gas. I would maybe get a compression test or leak down test done to make sure you don't have a bad headgasket.