By default, the pool will automatically adjust your miner difficulty over time to have 5 to 15 submits per minute. It's a good trade-off between bandwidth and accuracy.

You can also set a custom fixed difficulty using the password parameter. For example, if you want to set the difficulty to 64, you would use:

-o stratum+tcp://stratum.koinium.it:3433 -u username.device -p x

Here are the accepted values for custom difficulty:

Scrypt, Scrypt-N and Neoscrypt: from 2 to 65536

X11, X13, X14 and X15: from 0.002 to 0.512

Lyra2: from 0.01 to 2048

If the difficulty is set higher than one of the mined coins, it will be automatically reduced to fit the lowest coin difficulty.