Welcome! Electrolysis for the pipes. Phosphoric Acid ( Pool Cleaner) mixed 4-1 with water for the tank. Failing that, White Vinegar. You could also mix a tin of Black Treacle (Molasses) to a bucket of boiling water, and sling that in! Don't do the usual 'handful of nuts' to loosen rust from the tank insides, instead use a length of chain (not a bike chain), far easier to fish out than nuts!