Sounds like the right pump is pushing a lot more water than the left one. Things to check:
Are they oriented exactly the same? If the inlet is stuck against a wall it would cause limited flow. Also check to see if there’s something physically stuck in the inlet (algae, snail, etc)
As far as water level is concerned, adding water should technically only change the level in the rear chamber where the pumps are. If the display area is getting too high, then your overflow can’t handle the flow rate of the pumps. Make sure your overflow “teeth” are completely clear/clean. If they are, you’ll have to either find a way to restrict the pump flow (defeats the purpose of upgrading the flow), or cut out some of the “teeth” on the overflow.