True.....i think most of them contain some level of mercury, although some are spirit based, but anyway its probably not a good idea to break any of them in your tank i just know the mercury ones are more accurate from developing film for years, where a degree or two can make a difference, so ive allways used mercury ones, learnt that years ago studying photography in college.
The boiling point of water at sea level is 212 degrees Fahrenheit and 100 degrees Celsius. Since your tank is at 90 degrees and your fish aren't dead, I'd imagine that your thermometer is reading in Fahrenheit, not Celsius.
I think the best thermometor set up would probably be a stick on thermometor and an alcohol thermometer on the inside. Because mercury is poisonous. I think alcohol is too but too a much lesser extent. The combination of the two thermometors should allow you to get a fairly accurate reading. As for the speed of the reading, mercury may be faster but how fast does it really need to be. I can see maybe using a murcury thermo for water changes but not for in the tank, because if it breaks you'll probably have a bunch of dead fish.
errrrr duh i had a dumb moment last night before i went to bed...of course its 100 C for boiling point lmao.....yea i did mean its 90 F it has C next to it as well..... (tut tut my old science lecturers would be ticked off if they saw this haha opps! )
well i bought a new thermometer....and its says the same thing as my sticky on thermometer does althou...i have noticed that my sticky on one jumps from 86-90 on the scale and both are lighted up so maybe it isnt actually 90 degrees but inbetween which is showing up my new one also says 86 degrees...but the fish are fine!