What size are the small tubes? 10ml? 25ml in in a day or so is a lot. Or do you mean tanks by saying small tubes? EU tank would be 2ml so 5mls in a day or is very reasonable. If it's the 6.5ml tanks then that would be 16mls about average for a subohm by my standards.
You don't say which tank or coil you are running. I assume you bought it as a kit with the ELLO Duro tank which can use a 0.15 ohm or 0.20 ohm coil. The 0.15 ohm coil is going to burn more
juice than the 0.20 ohm coil.
Sub ohm tanks and conservative
juice consumption don't often bump up against each other in the same sentence. You might try another tank that is designed for MTL with a 1.5 ohm coil. That would drastically cut the amount of juice you are using.
Need more infor to really help you
What tank
Which coil - ohms
Clarify what small tube means in ml