What is to be done about a pharmacist who deliberately reduces the amount of water in the preparation of medicines to make them concentrated, thinking this is correct to avoid spoilage of excess quantities, and what are the Sharia implications of this?

It does not appear that you bear any liability for what you are doing. The excess solution is prone to spoilage and disposal, and you have only used what you deemed appropriate, without any infringement on the employer. What scholars have determined is that a private employee is not liable except in cases of transgression or negligence.