| Problem | Best Choice |
| Not leading from chest on underwaters? | Regular Eel-Fin |
| Underwaters not bilateral? | Regular Eel-Fin |
| Not finishing undulations in front of body? | Regular Eel-Fin |
| Undulations are just ‘knee kicks’? | Regular Eel-Fin |
| Full undulation cycle slower than 0.50 sec? | Regular or Pro Eel-Fin |
| Need to connect your core to your underwaters? | Regular or Pro Eel-Fin |
| Need to connect your core to your butterfly stroke? | Pro Eel-Fin |
| Need to develop more powerful recovery muscles? | Pro Eel-Fin |
| Need to fine tune tempo and speed of underwaters? | Pro Eel-Fin |
| Need to develop more powerful underwaters? | Pro Eel-Fin |
| Need regular method to work on underwaters? | Pro Eel-Fin |
| Want to build underwaters as part of cardio sets? | Pro Eel-Fin |
| Want a way to build your core muscles? | Pro Eel-Fin |