It looks like you have some room on the left - and even on the right. Is it possible to shift the sump over?
I was thinking if you shift the sump over, you could set up an ATO and connect it to a small 5 gallon salt container and place it next to the sump. This would resolve the issue with the uneven water level.
regarding the 3 mag 12s. You could replace two mag 12s and put in a single eheim pump (or even a single mag 18) instead. I won't talk about plumbing an external pump since your space appears limited already. You'll just need a T to split the line as it travels up.
I was thinking if you shift the sump over, you could set up an ATO and connect it to a small 5 gallon salt container and place it next to the sump. This would resolve the issue with the uneven water level.
regarding the 3 mag 12s. You could replace two mag 12s and put in a single eheim pump (or even a single mag 18) instead. I won't talk about plumbing an external pump since your space appears limited already. You'll just need a T to split the line as it travels up.