Some calculations to help you (or someone doing service)
500k fishing exp via minnow
Minnow provide 26.5 exp per "catch"
A catch can be 10-14 minnow so we will use 12 as our median
It would take 18,868 catches to get 500k fishing exp
18,868 catches translates to roughly 226,416 minnow
1 Raw Shark costs 40 minnow
226,416 minnow would get you 5,660 raw sharks
At current raw shark prices of 539gp each, 5,660 raw sharks would net you 3,050,740gp
At 90 fishing and perfect focus one can expect to catch roughly 19,500 minnow per hour or 45k fishing exp per hour.
If done with perfect focus, this service should take slightly more then 11 hours to complete.