You could try buying vinegar concentrate. It’s available in many bulk food stores and is 10x cheaper than in supermarkets. They usually sell 2x and 4x as it’s watered down for catering. It might say “acetic acid quad strength” or similar. Half fill a laundry bucket with this and soak the entire tank. We used the same method for rusty chain, just have to be careful to neutralise and seal immediately or rust will run rampant again.
Another solution is “rust converter” you can often get that cheap in sales at industrial suppliers. It’s safer to handle and some brands leave a paintable surface. We bought two 20 litre drums in a sale and it lasted for years. You could reusue it over and over again.
Molasses can also be effective but unless you’re on a farm or similar you’ll be unlikely to have that within easy reach.
