Def seam separation unfortunately… is the tank un level towards that front corner there? The wood canopy could be a factor also being heavy.
As for safety… if it was built with solvent (chances are it was) they usually dont just explode. It may let loose a bit and leak but in my experience it nothing like when a glass tank pops a seam so u have a bit more time to move fish, drain, and figure out ur next move. Ive personally ran a similiar looking seam for many years with no issues. I have also repaired/reinforced tanks with much less signs of stress.
You can repair/reinforce it with a piece of 1/2” square rod glued in with weld on solvent #3 or 4…#16 or the best option #40…. Or you can use #40 alone to create a fillet inside the seam. The 1st option can be done in place vertically. The 2nd option needs to be done on the ground with the tank tipped at a 45 degree angle to puddle the glue in the seam. Both methods can work well and are fairly “user friendly”…
My last resort for repairs like this are on the outside. You can get a strip of 1/4” acrylic 2”x24” and glue it on the outside of the tank covering the seam that way. This can also be done with solvent, #16 or #40.
Personally i think the fillet of #40 can look the nicest. Square rod can look good as well if ur clean with it. Being the front id almost just do the other side as well so it looked symmetrical but thats not necessary.
For starters you can lower ur water level to elevate some of the pressure as well as remove that heavy canopy until you decide what moves to make for the fish and repair.
This is nothing out of the norm for an older acrylic tank. Or even a new one. Things happen and they can be fixed. Its a fairly minor repair and the tank cant last another life time afterwards
