So I bought an 95 zx6r ninja stock with 6000 miles last week. I noticed that the fan does not turn on period. While riding obviously its no big deal, but in traffic I have to shut the bike off sometimes. It has gotten to red sign, then I shut it off. Today I was riding all day and when I shut it off and put it in the garage, I noticed coolant was draining from the overflow tank; it was filled to the top with coolant.

That's a wide open answer. Is it totally stock with the crossover pipe in the exhaust ( 95)? Do you have two into one racing pipes ( 110)? Do you have drag pipes ( do you want it to idle or run good wide open throttle?)(125) ? Do you have stock air filter? Stock ignition? Dynojet kit?
The correct answer is to tune it. That means: put it together and ride it for 30 miles ( unless its obviously way off) and check the plugs and adjust and ride it for 30 miles again and check the plugs. Once you get it close you can adjust with idle air screw but my limit is 2 and 1/2 turns out. You get past that you need a bigger jet.

Have you checked the wiring for the kill switch?

The overflow is a result of the constant overheating, check your fuse and your fan relay, if those are both fine, then wire your fan direct to a power source and see if it comes on if it does your last spot to insoect is the temperature sensor located in the radiator, its going to be one of those things for sure

you need to do a compression check before you do anything

With the 05 I would check the thermostat. With the 03 that is normal, my 02 cuts on and off around the same range. I put a switch in so I could turn my fan on when ever I wanted. I run mine once I get to 175...

when i ride my bike and i crack down on it it boggs down its a 96 zx6r

no it will rub on the swinging arm

Hi, Thermostat yes. That is the next troubleshooting step. Good Luck!