Here is a tested Arduino project that uses 3 sensor probes as water level indicator with 3 leds and one LCD display + a simple controller that turns on a motor when the water has reached the desired ...

It runs on an Arduino MEGA 2560 PRO (or similar microcontroller) and supports a wide array of additional hardware including pressure transducer, water level sensor, flow meter, OLED display, and more.

When I drain water (water change) I use a ‘U’ tube and even if I am not there it will not drain below that level. A full drain of the tank would need to transfer the fish to another tank.