By itself- I think it wouldn't seal. Too flexible and sealant might not stick.

However, if you get a thin piece and use it as an inner liner for a plywood top, the plywood would add rigidity. Butyl tape would be perfect for that application, great gap filling and stays permanently flexible, and easy to remove when needed. But if you drink your tank water, butyl tape might not be drinking-water safe. You may want to choose a different sealant. We use bottled water for drinking and tank water for washing, so in my case I wouldn't worry about it.