Authorities say the toddler was standing on the tank's lid when it collapsed underneath her, causing her to fall 15 feet Wednesday.
She was with her mom and siblings at an RV park, visiting her dad who works in the Aransas Pass area.
Witnesses tried to rescue the girl with a rope, but it was too short.
After crawling through a two-foot hole, firefighters were able to retrieve her body an hour later.
Police believe the girl died from the fall, not from drowning.