A little bit of topic ...
I have of these TL-136 (0-1m) pressure sensors for my pond on my desk (they are available in different ranges). They are a bit pricy, but should work fine.
I tried to use the water proof ultra sonic sensors too but they have a minimum sensing distance around 0,4-0,5m - which is a game stopper for me. A non waterproof ultrasonic sensor died quickly cause by the harsh outside conditions. Also I have all the parts (ping pong ball, pipe, transparent case for a DIY ToF range sensor VL53L0X - Time-of-Flight ranging sensor - STMicroelectronics ... but no time
If you have access to the outside of the tank a pressure sensor like these
should work. I use them to supervise my pumps. They should work not only with water (Tip: Be carefull with electrodepotentials - one of these corroded away in weeks connected to the metal pipes of the pump where others isolated by plasic pipes look as new 50cm downstream after 3 years.
I only need a range of 0-10cm to make sure that the skimmers work as intended and the balanced system with 3 connected ponds is not blocked to avoid that the circulation pump drain some ponds downstream while overfilling the ponds before.
