Sight glass in coolant tank leaking

You've received good advice already so I won't repeat all of that. Some sight glasses are glass and some are plastic, really makes no difference which material, but be careful removing the one you have. From personal experience I had one crack at the base and leak. When removing it the threads snapped off and I had a pretty hard time getting the broken part out while on the road. No easy out in the tool box. There is a O ring seal and using a little teflon tape or paste on the threads makes the part a little easier to remove if you ever need to again. It also provides some added leak prevention although that usually isn't needed.