I posted a comment on the Concept 2 UK forum about Empty the tanks and I was pointed in the direction of this website http://c2ctc.com/.
This is a virtual cross team competition and the May challenge is for your fastest 5k time. The principle seems to be that members from virtual rowing teams can enter 'boats'. There can be 5 members in each boat and the average time of each rowers 5k is calculated and then entered onto the board. I have entered a boat already, so I need 4 more rowers to join me - I am currently the 77th boat out of 79!
If you want to join me, go to the link above and you will see 'Add or update a person' in the top right hand side of the page. Add yourself in this section (don't forget to select Empty the tanks team), then go back to the main page - you can now enter your fastest time for a 5,000 metre row to join the rankings.