Hard water is high in minerals. You can recognize it if your faucets are stained white with use, or if you boil water and your pot has white marks. If this is your case, you should use biodegradable powdered detergent, as it contains softeners. You should dilute it with water before adding it to your diapers.
Soft water does not leave this residue so you can use liquid detergent if you prefer.
Now that you have identified your water type, choose a brand of biodegradable detergent of your choice, one that is easy to find and within your budget, and follow the manufacturer's instructions to calculate the correct amount. Use the equivalent of the amounts per kilo of clothes or diapers to be washed; remember, diapering is "Trial and Error." Adjust the detergent measurements as many times as necessary.
Choose a brand of biodegradable detergent, try it out, and avoid changing it, as this could cause problems in the future due to mixing detergents. 
If you are still unsure about what type of water you have, using powdered detergent that contains softeners will always be the best option. 
If you have the opportunity, always try to use special detergent for diapers, since they have the exact measurements, and are designed especially for them, contact your authorized distributor. 
