@subzero
i have 2 tanks each with a volume of 50,000 litres. They are made of normal machine cuts stones well done by a tanks fundi. They are sub-underground ( 60% under and 40% above ground) The following tips would help,
1. Avoid using fundis who build houses to do for you a water tank.
2. Get a water tank fundi with good references. Most good ones have references from Kenya Rain water association.
3. Avoid myths like hard stones are the best, use water proof cement etc. All that is important is the building technique and the skills used.
4. Ensure a good gutter system is done to both ensure first rains water is drained away and only clean water gets to your tank.
Hope this helps
"Things that matter most must never be at the mercy of things that matter least." Goethe