I should probably say that I only have a layman’s understanding of the physics and electronics involved, all based on a few google searches regarding temperature and [water density, water volume, and pressure sensors]. Luckily I added a BMP280 sensor (thinking I may need to compensate for barometric pressure). When I noticed the unexpected fluctuations, I added the BMP280’s data to Grafana and was first able to realize the issue and then gather the data to write the compensation in firmware.
What you described seems to be in sync with the thermal effects on water. My readings weren’t, which lead me to believe that it was the sensor being affected. Either way, if you’re experiencing fluctuations that seem to coincide with the sun, it is likely a temperature issue.
As your sensor is the drop-in type (same temp as water) and the readings seem in sync with thermal effects on water, I suspect you need to monitor the water temperature in your tank. Having my sensor and other electronics all external and housed in the same environment allowed me to use the BMP280 sensor. I can’t recommend any particular in-tank water temperature sensor.
Add a temperature sensor internally (in tank) or externally (at microcontroller), and collect data on a fixed volume of water over a few days. Once you add temperature to your graph, you can verify if a spike in pressure corelates to a spike in temperature, and by how much.
The links up top should help understand the physics.
This is my tank and sensor setup:
