90 gallon fowlr

[daplatapus]

I read a lot on auto water changes before I decided on mine. What you have posted is doable, but would have to be monitored extremely closely. Which IMO, defeats the whole purpose of an auto change system. Take for example a couple scenario's which could potentially go wrong.
1) your waste water pump isn't EXACTLY pumping the same as your new SW pump. This will cause your salinity to drift. Either by taking too much out, making your ATO pump just fresh water in, or taking out too little, and filling your sump higher than normal. This could potentially mess with your skimmer.
Film buildup, or a myriad other factors could potentially cause one pump to no longer equal the other.
2) your away at work, vacation, etc. one pump fails, or for some other reason doesn't turn on.