Your void setup need to initialise a BlynkTimer that will call your updateTankLevel function at the required interval. Because that’s missing, your BlynkTimer is doing nothing.
Read this for a simple example…
If you want to understand more about BlynkTimer then read this…
You haven’t taken onboard my comments about the log event code, so testing of notifications won’t go the way that you hope, and you’ll need to wait 24 hours after running the sketch to allow the event limit to clear.
Having this in your void setup probaly won’t produce the results you’re expecting either…
Because it takes time for the Blynk connection to be established. Depending on exactly what it is that you’re trying to achieve, you may be better reading the document in for BLYNK_CONNECTED()
It depends on your relay. Either way, you’d be better powering the relay from the 5v pin on your board rather than the 3.3v pin, as it’s easy to overload the onboard voltage regulator which converts the 5v supply to 3.3v.
Pete.