Is BluTack safe to use in an aquarium? What other such stuff is safe to use?

I wouldn't use it.

use something like silicon

Make sure if you use silicone it is the fish freindly type, maybe even a hot glue gun?

bluetac doesn't really stick underwater- they say it does but... hot glue gun glue is not very good if you are glueing something to the glass- as it doesn't readily stick to the glass- it does work for sticking other things together- rocks, pipes etc- although I don't find it as strong as silicone. I use sealys glass and have had no problems with it.

What about aqua kneed it? it sets hard though.

either aqua need it or chewing gum.

is hot glue safe to use and would it be strong enougth to hold the coral/rock on top of each other?

I have personally used blu-tak to hold down some veggies so my plecos could eat them. They were perfectly fine. The fish do not get poisoned or anything. Some of them tried to graze on it but it was harmless. It is true that it doesn't stick in water well. It just keeps coming off. Blu-Tak is fine for fish. It even says untoxic. But I would avoid the $2 shop ones. Trust the original ones.