Here's a way to clean an old tank - first empy everything out and refill with fresh watr, add 2 tablespoons of bleach for every 4 litres of water. Let it sit for at least 30 minutes. Emply the tank, rinse it well and then refill.To neutrakise any bleach residue, add a chlorine neutraliser available from pet shops. Empty the water once again and rinse.