A classic example of what happens when you store too much water.
Inadequate turnover allows the water to stagnate and bacterial films can result. The water can harbour pathogenic bacteria such as Legionella and the taste and smell can be affected.
The solution is easy. Clean and chlorinate the tank and then reduce the amount of stored water.
Water Hygiene Solutions Ltd are experts in the remediation of problems such as these and can chlorinate any size of tank leaving it clean and fresh.