retrofit
Will it blow up or melt? First, you can't just put in a 65 watt bulb. If you put a 65 watt light in a strip that held a 20 watt light, you will be doing a retrofit. That is, you pull the guts out of the light, put in a new ballast and stuff to hold the (different looking and shaped and end parts) bulb, rewiring it.
Now 65 watts of power is more than 20 watts of power, so the new bulb is hotter. You will want to add vents or fans of some sort. The AHSupply retrofit kit has pop-in vents that do a good job of letting the heat out (not sure about other brands).
If your fish are sensitive and heat build up is an issue, you may want to either elevate the lights a bit and/or add an actual fan to the hood. Just ading a small wedge of something to lift the lights 1/16" will help the airflow a lot, if you added those passive vents.
In fact, you will probably need to remotely mount the new ballast as it is the source of a lot of the heat. If you just add the ballast to the outside of the strip instead of the inside as it is now, that helps, but the light is still putting out a lot of heat alone, so the vents and fans may still be needed.
Now, if you have done a retrofit, and the ballast is inside the old strip and you didn't add vents or fans or elevate the lights a bit... you might melt a plastic light fixture. You'd see a lot of heat in the fish tank first, though.
