Track only decreasing values in tank usage in current day

I am trying to track how much has gone out my water tank.

I am using InfluxDB as my Data source and Flux as the query language.

I can easily find how much is used so far today by using the difference between the first and the last value in today() as a time range.

But if I try to use this method on a day that I fill up my tank it all goes off as there is more than when it started.

Is there a way to only track and add values when the value decreases ?

This will give me how much has gone down in the day and not how much it has gone up.

I contacted the Influxdb community and they helped answer this problem, here is the link if anyone else needs this.