Hey Hey Hey

ive found some sites that say it will be 36 inches
Snowflake Eel (Echidna nebulosa)

Snowflake Eel Profile - Echidna nebulosa Moray Eel

so dont be surprised if its gets bigger then 2 feet. dont always trust what one site tells you, as they may just be trying to make a sale. i doubt most will make it to 3 feet though as very few fish survive there whole lives in aquariums, also most fish in aquariums lack exercise and a proper diet. if you provide him enough food i would expect him to get atleast 30 inches. just so you know i would consider 8-10 inches of eel a kinda big bioload for a 20 gallon since its a predator. get some killer filtration, overated skimmer etc. im just warning as i would hate for your first foray into salt to fail.
dont be afraid to ask your brother for advice.