Clean water turbidity sensor
I'd suggest you are limited in resolution by the distance through the water sample with your current sensor. The datasheet shows you have no more than 5.7 mm of water depth providing resolvable transmissive information.
 You are also severely limited with the sensor since it provides only transmissive sensing without any backscatter sensing.
Perhaps you could make you own sensor from two flat glass plates and install multiple mirrors to bend the sensing path through multiple passes through the water. This would give a longer path but still lacks any backscatter sensing. 

This may be of help ...have a look at the GLI Method 2 sensor.