Several years ago, I was newly exploring observance, and at the time used glass candlesticks for Shabbat. I placed a small amt. of water in the receptacle of each candlestick, so that the heat of the fire wouldn't cause the glass to crack, but after a while, I had the uneasy sense that there might be some objection to this. I then learned that it is forbidden to put water under Shabbat lights. (I have forgotten the exact reference.) Recently, I have been with a rabbi who puts oil and water in neronim cups, and I asked why oil was different than candles in this respect? (Or maybe it is not.) Does anyone here know?

By oil, your reason to do it is to lift the oil, or that when the oil is finish, it shouldn’t burn further, so it just causes it to go out. By the wax candle, the flame actually falls into the water and extinguishes it. See OC 265 MB 18. See there that in case of great need, you can rely on the Shita that it’s only prohibited if the water is in a separate utensil (that’s placed underneath it to catch the sparks.)

Thank you.