The tunnel is small just few meters and not interesting. just a straight line.
There are different parts of the aquarium you can visit on extra payment.
boating, scuba diving, the tunnel and the fish zoo. But for me it was an OK experience. fortunately my ticket was free so i didn't bother it much.
watch it from the outside only for free.
or if u have kids, they would love it.
Remember: different tickets for different parts.