IMO velcro isn't a good option because fuel soaking can make it loose. Also cold weather often makes the velcro glue super weak.

The best way is to design, glue and cut some tabs or half formers and bolt the tank to the fuselage. Our jets see pretty extreme Gs and a moving tank can compromise the CG and as a result, the controllability of the model.

You will need to sand the tank and clean with isopropyl alcohol before gluing anything. Hysol is great for that operation. Any surface sanded needs to be checked for leaks.