I have read a lot on these nano sized tanks on various forums and have not ever seen or heard of any of them catching fire. There are various problems associated with various brands and it seems each brand offered has its own brand of poison associated with it, but none are fire related...
The biggest concern with your 12DX is cracked glass. Make sure you do not have any minute scratches or cracks already there, just waiting to propagate further. If there are scratches, keep a close eye on them, not that there is anyhting that can be done, about them, but just sort of watch them to see if they turn into a crack.....Biggest polace they start is at the front corners. Make sure the unit is setup on a flat planar surface. Have fun.........if yu do not have any cracks aor major scratches and its a used tank, I would feel pretty confident in its integrity as most usually broke within a few months or even days.
NANO by JBJ nortorious for cracked tanks, and poor pumps, noisy fans
AQUA Pod by Current......initial wiring problem (now totally recalled and fixed) not any problems with glass other than broken glass during shipping, and some bad pumps and noisy fans
BIO CUBE by Oceanic, noisy fans and water level height regulaton