How to choose a perfect resistor to LEDs to get same brightness?
The trick is not in matching resistors but in matching current, since even the same model of LED has differents Vf among parts and the resistors are computed over that.
This, of course, if you have the same model of LED. If these are different (like different colours) you need to look up the curves for the light emission. If you are lucky your leds come from a 'companion' family (i.e. red/yellow/green leds build to go together in a scale).
How to give them all the same current (or the right current)? Well that's depend on your circuit topology.
Using comparators there a neat trick involving all the leds in series and a single current source. Look for &quot;THAT Corporation Design Note 112
LED Bar-Graph Compression Indicator&quot;, it's for audio but you can trivially adapt it to your water level.
If you are using some kind of digital control there are dedicated ICs to match the current of indicator leds. Some of these even have some programmable register to match different LEDs. Look in the catalogues, TI and NXP make a lot of these.