To find the cause of the water loss, one would need to monitor where the level of the water is when the water level stops dropping. This will be the level of the lowest leak in the pool. The water level will normally stop at one of a few distinctive points as follow:
Cracks in the concrete shell of a pool do not necessarily cause water loss. Cracks smaller than 0.25mm in width look bad, but very seldom cause water loss. Although they might grow at a later stage depending on the cause of the cracks. Cracks bigger that 1mm in width is considered structural cracks. These can surely cause water loss and normally grow larger and become more problematic over time.
These cracks found in fibreglass pools, can only be permanently fixed by installing a new fibreglass lining of the entire pool pool. Such cracks in a marblite pool are caused by wrong steel re-enforcing during installation and cannot be corrected without recasting a new concrete shell. With a marbelite pool, major cracks can be chiseled out and patched with varies methods, but the crack will be back after about 6 months, leaking again, and resulting in a color difference in the finish of marbelite. Cracks in a fibreglass pool are accompanied with or without bubbles in the lining. They can be patched but we strongly recommend not doing so as the concrete has been extensively soaked as the problem is normally not picked up in time. The concrete cannot offer a proper bonding foundation for the fibreglass and will 9 out of 10 times pull loose again within a short time after the patching has been completed. This is most likely caused from the concrete behind the old fibre lining not being able to dry out.