CO2 tanks are filled by weight, not pressure. Any liquid CO2 in the tank will result in the same pressure, but the weight tells you exactly how much CO2 is in there - which for your tank, should be 2 kg above the tare weight.
The best leak detector fluid I have found is a mix of water and liquid dish washing soap. The consistency should be that which is used for a childs bubble blowing toy. You can liberally soak everything with that, leave it alone for several minutes, then look for mounds of little bubbles where the leaks are. Big leaks are really very tiny leaks, so don't expect to see them easily.