Those look like stress fractures small cracks that will become large crack over time. your best bet is to buy acrylic glue lay the tank down with the cracks facing up and spread acrylic glue evenly over and let dry then sand smooth with 2000 grit water sandpaper soak the sandpaper in 1 gallon of water with a 1/2 teaspoon of dish soap sand smooth then run high speed buffer at 1500 rpm with buffing compound and again with foam pad and finishing compound. Just what I have done with plastic in the past also if in the end you scratch it up to much after sanding you could spray the outside with a clear acrylic enamel (car paint) and bam no scratchs I've done this with a set of headlights and worked great.