I'm pretty sure the red fluid in the aquarium thermometer is just red dye and alcohol plus steel balls for ballast in the bottom to keep the thermometer upright.
Just make sure you get all the glass shards removed and use a magnet to find the ballast material.
I've had a ton broken in tanks. They don't cause issues. They do leave annoying little beads everywhere that small fish who like shiny things seem to think are food...