Hi, and welcome. I too think your tank temperature is too high. I like to keep mine right under 80. And once it is set the trick, with a 10 gallon, will be keeping it from changing. If you keep it from changing you will keep the fish from stressing out and if the fish aren't stressed they...
First a 75 is a little small for a tang, the rest of the fish should be fine, but if I were you I would screen the top of my tank...any fish has the ability to jump...especially a goby.
Next time you do a water change just put water back in that's somewhere around 1.020 or a little lower...and keep doing that every time you change your water until you get the level where you want it...
It's a scaredy fish (gets scared easily)...and my guess would be that your clown scared it...especially in a 12 gallon...that's not a lot of room for both of them to have their own space...