Splitting of tank seams is usually due to the tank being placed on a "bowed" surface causing tearing of the silicone in two opposite corners. Are you sure your stand/tank is on level floor ? My mates tank split down one corner (and started on opposite corner) due to one of the legs on his "chest of drawers" tank stand collapsing, causing bowing of bottom of tank and subsequent tearing of the silicone. He made a better stand and resiliconed tank and that was many many years ago and tank is still fine.
I'll check tonight, It's the standard fluval chipboard stand so I assumed so.. the floor is in a new extension so pretty flat
Did your friend completely strip and rebuild the tank or did he get away with just gluing on the front panel?
I'm still amazed that the fluval is only 5.5mm all the quotes I'm getting for a similar size tank are 12mm thick glass