You don't put water inside the diffuser!
That diffuser was probably on a timer and had been switched off..... and water has seeped into the chamber!
Best if you soak the new diffuser in water for a few hours this will help to let the Co2 out!
75....Will be far better for a 60 litre!!!
75 is pretty powerful.....I have one installed on a 35 Litre Denerelle Scapers Tank.!
I increase the flow to the max...every now and again, just to remove some crud on leaves and detritus....but it is like an old fashion upright washing machine...
Used one in this journal....you can always keep it as a backup if you run out of Fire extinguisher/cylinder Co2!